How do you compare chainsets on MTB? Which is better?

First off, the higher end stuff is lighter. SRAM SX is built extremely cheaply, it is worse than Deore by far. Much like how cheap bikes are known as "bicycle-shaped objects", SX is known as a "groupset-shaped object".

You also need to take into account your riding style: just how much gear range do you need? Seeing as you intend to ride on the road quite a bit, I would definitely go for a 2x system.

So why is 1x better for MTB?

1. The shifting pattern is much simpler. You don't have to to worry about cross chaining or redundant gear ratios or any of that. It's simply "bigger number = faster, smaller number = slower". With a 2x or even worse a 3x, it is easy to lose track of which gear combo you're currently in. It's also less fun to be constantly thinking about gearing.
2. Having exactly the perfect gear ratio is not as important because of the constantly changing terrain. You also don't need to pedal as much on the downhills, so really all you need a drivetrain for is the climbs and flats.
3. Better frame design is possible without having to accommodate for the front derailleur.
4. Lets you run an under-bar dropper lever.
5. Lighter weight and more reliable as it has fewer moving parts (no need to adjust the FD anymore too). The "narrow-wide chainring" and "chainguard" are two examples of devices you can use on a 1x system to stop the chain from falling off as much.

You are mixing quality with quantity (on this case gear range).

1x in a true MTB setting gives more than enough range for a majority of riders most of the time. MTBers on 1x tend to elect a chainring that gives the lowest low gear they need for climbing, and accept the limited loss of time that occurs the few times they are spun out at high speeds. Reality is if a technical MTBer is going fast than spun out speed, they are concentrating on lines, jumps and landings, speed is far from a priority. The advantages (listed in the answer by @MaplePanda) out weight the loss of top gearing.

2x is useful for those who ride big hills on roads and need a low gear for climbing and a high gear for high speed descents, or weaker riders than need super low gears for hills. Until recently 2x has also been a viable choice for budget, but 1x is now avaible as some quite low price points. The 1x fanboys often come with a 1 eyed view of its advantages, most of which suit experianced rider significantly more than inexperiacned riders (eg. Anyone thinking about Altus or SX is not a weight weenie, probably cannot afford a dropper and almost certainly buying a hard tail where there is plenty of room for a front derailluer)

There are many other group set options on bikes - if you cannot find what you like at the price point you can afford, keep looking.

In terms of quality, Deore is (IMHO) the best in the list you have given. Altus is considered too low quality for reliable operation MTB standards, but is suitible for road and occasional light offroad use. SX Eagle is somewhere close to the Deore in quality - probably comes down to personal preference.

Might I suggest two shopping options you may not have considered?

1. Buy a higher-quality used bike, say 5 years old, and have it thoroughly checked and prepped by a bike mechanic. You should be able to find a lighter, better frame and a good quality 2x drivetrain (or whatever you want).
2. If buying new, see if the bike shop will swap out stock components for those more to your liking. For example, a better quality rear derailleur (my Shimano rule of thumb is Deore or better). There are plenty of quality "NOS" (New Old Stock) components available from past model years, so a bike shop should be able to find and install (e.g.) a new Deore 2x drivetrain in place of a stock Altus. The shop will then later sell the "take off" components, so you should get somewhat of a discount on the NOS components or their installation.