Heavier or lighter?

I’m a 50 year old man who’s having to get himself a bike for daily commute to my workplace so as to be able to avoid taking public transportation and being able to travel while being able to keep myself socially distanced. I will be mostly riding on flat roads, which are not amongst the best road that exist. I also have this condition named osteoarthritis, that causes me knee pain, and the doctor has suggested that I should either ride a bicycle or swim. And if I do either of those suggested activities, the pain on my knees gets reduced,not increase. A few days of regularly doing either, and the pain goes away totally, but comes back if I stop. I did swim every season and it did help but it didn’t happen this season because of the ongoing pandemic..

I have shortlisted two options, both of them are very similar, apart from the fact that one of them is a 21 speed and another is an 8 speed. I really don’t know how much of either I’d be using. Where I’m more caught up is the difference in weight of both. The 8 speed one weighs 32 pounds and the 21 speed one weighs 39 pounds. Addition of mud guards, pannier, side stand will add more. Now, my commute will be about 22 kilometres (11km up and 11km down). There’s a 15 km/day limit for over-use on the 8 speed bike, which is from an internationally renowned brand and no such thing for the 21 speed one, which is from a local brand. Both of them are made of "lightweight steel". I would prefer to support the local bike shop, but I have to understand what’s better suited for myself too.

If both bikes are made of the same material, which they most probably are, does the added weight for the 21 speed bike give more durability too? Since I’d be spending more or less the same amount on either I choose to buy, it’d help to know which one would probably last more..

So all in all, is it a good idea to buy the one that’s heavier?