First road bike: mech disc brakes vs dual pivot sidepull brakes?

I'm going to focus solely on the issue of mechanical disc versus rim brakes.

All else equal, I would prefer rim brakes to mechanical disc. Mechanical disc brakes (and the hubs, and possibly the frame as well due to increased manufacturing complexity) are more expensive than rim brakes. This means that if the bikes have the same price, then the one with disc brakes would have more compromises elsewhere in the spec (on average).

In my (admittedly limited and second hand) experience, mechanical disc brakes have relatively poor stopping power out of the box - one bike I helped someone buy stopped more poorly than my bike with good rim brakes. Mechanical disc brakes need good compressionless brake housing to work well, which entry level bikes don't come spec with. Also, they need more frequent adjustment than hydraulic disc brakes. You have to adjust the position of both the inboard and outboard brake pads as the pads wear down. With rim brakes, you just turn one barrel adjuster. With hydraulic disc, the brakes self-adjust.

For cheap rim brakes, the compromise is that the brake pads are often poor quality. This is very easy to rectify. While we do state no product recommendations in the FAQ, we do break this rule to recommend Kool Stop brake pads, which I believe are available in the UK (it's a US-based company). If you feel your rim brakes don't brake adequately, have a store change the pads and holders first, or do it yourself.

If you get more interested in cycling and want a higher-spec bike, chances are that you'll later sell off the bicycle wholesale. It's almost always more cost-effective to upgrade the entire bike, rather than upgrading piecemeal. I would wager that mechanical disc brakes are no better than rim brakes of a similar price point. In contrast, on a much higher-end bicycle, hydraulic disc brakes do objectively have more stopping power (especially in the wet) than rim brakes, so in other contexts I would usually recommend hydraulic disc over rim brakes at a similar price point.

Terminology: you are looking at a bike with mechanical disc brakes. These brakes, as well as traditional rim brakes, are operated by a steel brake cable. In contrast, hydraulic disc brakes are operated by hydraulic fluid, e.g. mineral oil or DOT fluid as used in motor vehicles. One of the bikes you identified was equipped with Shimano Claris, its entry level road groupset. On Shimano, hydraulic disc starts with Tiagra, which is two levels up from Claris.

While specific shopping comparison questions are off-topic here due to their tendency to become obsolete, there is a perfectly good generic question here where these two bikes can be used as examples, so I will take that angle on it.

The two bikes in question have similar price points, but the braking system on the disc one is eating a lot more of the price tag. To compensate, it's a 7-speed freewheel bike in 2021 and has essentially the worst road bike components money can buy, and the unnamed brakes that are getting room made for in the price tag are probably themselves very mediocore. This dynamic is common with bikes: there's a must-have feature that stands out to consumers, so in order to fit it in at a given price point, everything else is downspecced to the point where the overall level of functionality is arguably worsened. Product managers who attempt a more balanced approach at the entry level price points, where there exists a greater amount of ignorance to be exploited, are punished for their integrity and don't stick around the business. The sidepull bike in question is more in that direction: something someone who knows about bikes would choose for themselves if that was the budget they were working with, but which lacks features deemed essential by a hype-driven market. (Few low-end road bikes are pure examples of this because of the massive impact of brifters on their price tag.) So if you do get it, you will have to live with the fact that you've chosen a bike with the wrong look and bullet-point features, and your only consolation will be that it's saner and better.

It is true that disc brakes have advantages for wet weather conditions, but people rode in those conditions avidly long before them. Meanwhile, getting and keeping bottom-end mechanical discs in good adjustment can be challenging for the novice, even assuming the brake in question is a good enough one where it's even possible (not a given), whereas sidepull caliper brakes tend to be much more approachable and easy to deal with in terms of common pad and cable type adjustments. It is very common for cheap disc road bikes out in the wild to be found with barely adequate or inadequate brakes.

Disc road bikes of all kinds, cheap or otherwise, have the advantage that they can take mudguards/fenders when a similar sidepull bike couldn't, or not with the same tire size. This may or may not be important to you, but it's a factor to consider. Contemporary cheap sidepull road bikes do often have "standard" aka 47-57 aka not short reach calipers that are capable of taking them if the tire is small enough and the frame/fork take advantage of the reach.

Good mechanical disc brakes (e.g. Avid BB7) can be very good indeed - plenty of stopping power, pads last for thousands of kilometers, not affected by weather. However, I've had varied experiences with cheaper mech discs, such as some Shimano models. The biggest problem seems to be a lack of stopping power when used with drop-bar levers (even though these were supposedly drop bar -specific brakes). On a budget priced bike, I'd be a bit suspicious about mech discs. At the very least, I would test them carefully before buying.

This is quite a wide-ranging question with a number of points raised. Regarding the specific question of mechanical vs. sidepull, I can only agree with the previous commenters that you are likely to get better value and experience from the sidepull brakes, from my experience they are much easier to keep 'in tune' than disk brakes and will almost certainly perform better £ for £. I would also endorse the suggestion to upgrade the brake pads as this can be a very cost-effective performance boost. There is a lot of advice on forums and websites that can inform purchase choices. Nathan has also made some excellent points about the overall value proposition for both bikes.

I would like to add that one of the most critical factors in ensuring a successful bike purchase is fit. Given that this seems to be your first purchase, you will not have a point of reference for what works for you in terms of bike sizing. I am not sure of the current status of 'try before you buy' given we in the UK are in lockdown, but both Halfords and Decathlon offered this service for any bike they have in store; I would strongly recommend getting a feel for what fits before committing to a purchase. I have no personal experience of purchasing a bike from either retailer, so I can't comment on after-sales care but I would always recommend considering your local bike shop for repairs and maintenance you can't do yourself.

Bike fit is relevant to the question you raised around the secondary brake levers offered with the the Triban. I have used this style of brake lever before and I have found that while they seem appealing they are something of a solution looking for a problem and in my experience reduce the effectiveness of the entire braking system, whilst increasing overall complexity. To expand on 'a solution looking for a problem': if you get the fit on the bike right you should naturally want to ride with your hands on top of the brake lever hoods rather like this chap from Decathlon's own website which will provide adequate brake modulation at most speeds. And they certainly clutter the bars up when you are trying to mount accessories!

On the theme of bike fit, it is worth considering widening your search away from looking at bikes that are not specifically marketed as "women's". Assuming that an appropriately sized frame is available, bikes are unisex with the exception of the saddle, which can be swopped very cheaply. While there is no hard and fast rule about what works for a given individual the spacing of the 'sit bones' tends to be sufficiently different between men and women to require a different saddle design. There is a lot of advice out there about both bike fit and saddle choice, for example this article. The reason I mention this is there are a number of options out there such as the Brand-X Road Bike from Wiggle/Chain Reaction that meet your budget without some of the compromises of the linked bikes.

To sum up, they both look like reasonable bikes at the price point with some compromises. If it is possible in current lockdown, see if you can get to a store and try one out so you can get a better idea of what fits you well. Consider trying other bikes that might not be specifically marketed as 'for women', as it is a trivial and cheap exercise to swop the saddle out for something that works better for you personally.

A good rim brake system (specifically, linear pull/V-brakes and road, dual pivot caliper rim brakes) equipped with quality pads have very comparable performance to disc brakes, mechanical or hydraulic. When the associated higher costs of disc brakes, in both complete bikes or aftermarket upgrades, are evaluated against the very minimal advantages gained over rim brakes, the value of disc brakes becomes questionable. When continuing maintenance costs and complexity of disc versus rim brake systems are included in the eval, the value of disc to rim brakes falls precipitously. Any questionable quality aspect effecting performance, set-up, or ongoing maintenance of a disc brake system drives their value further through the floor.

The ridiculous propensity of disc brake pads to become contaminated, often resulting in loss of the prime level of performance and creating irksome noise leading to the need for additional maintenance time and costs, is infuriating. The industry's nearly complete move to disc brakes without viable rim brake options from which to choose raises my hackles, since these issues were surely considered. The very poor cost to benefit was clearly evident to the manufacturers, likely in a more detailed (and damaging to the cost-benefit ratio) elucidation of the issues faced by a disc brake system for bicycles. Despite this, they decided to not only continue this farce, but also force-feed the consumer the disc brake folly by eliminating rim brake options while seemingly ceasing any meaningful advancement in addressing performance issues as mentioned above (and several others). Today, there most certainly continues to be no change to the poor cost to benefit of disc brakes.

I've read so many different opinions on mech disc brakes vs dual pivot sidepull brakes but I'm still not set either way...

It sounds like you have a good understanding of the pros and cons. As others have suggested, the rim-brakes are likely to represent better value overall and are easier to maintain yourself. If you find that stopping power is not as good as you'd like, new brake pads (preferably with separate holders) are a reasonably cheap upgrade.

However, Halfords appeals as they offer lifetime bike health checks, warranty on the frame and Carrera seem to be a decent brand for a beginner?

I think "health checks" means they'll look a the bike for free and tell you how much it will cost to fix.

Consider visiting your local independent bike shop for maintenance and a hopefully honest discussion of your needs. You may also be able to physically try out a new bike, which is an important part of deciding whether it is right for you. You can use this website to find your local bike shop, although I notice that my local ones are missing.

[Triban] I'm thinking when I get more into cycling they may become an annoyance/no room for lights etc?

I think you could have those removed later if you wanted.