I like Elixir Nanoweb .12s for my acoustics, but I try a lot of different strings. D'Addario, the company that makes Elixir (I forget their name), and GHS make their own strings. Pretty much all other strings are made by three manufacturers. Given that, there are some differences. Just because the same company makes lots of different brands doesn't mean that they don't retool or use different materials for their manufacturing runs. It does mean that quality control is going to be pretty consistent across brands, and the QC is pretty good.
The reason I like the Elixir Nanoweb .12s is that they last and hold their tone. If I put on Silk and Steel, I get maybe two weeks. With Elixir Nanowebs I can easily get two months. I also like D'Addario Phosphor Bronze for my acoustics, but again don't get much life out of them. As an experiment I've hot D'Addario EXP's on Boomer (my bigger guitar), and they've lasted as well as the Elixir Nanowebs. Tiny, my smaller guitar, has got Peavey Phosphor Bronze on it. They came out of the cheap bin. They're perfectly good strings. They're worn out now, but they've lasted a decent while, sounded pretty good when fresh, and only cost $3. I've got several sets of them in my guitar case as back-up strings.
I've bought some Ernie Ball acoustics, and also I think some sort of rare earth thing that's been frozen or some kitschy thing. They were also from the cheap bin. I like messing around with different strings.
As far as flat-wound, I have not tried any flat-wounds on my guitars, but I know they are available. The difference is likely to be in feel rather than in sound, but they are likely to be expensive. As quickly as I go through guitar strings, I'm not going to pay for that luxury. If it is a medical thing and the price you have to pay, fine. I'm not sure what arthritis has to do with finger tip sensitivity, but if you've noticed a difference for you then it doesn't matter what I think. I do use flat-wound strings on my fretless solid-body bass. Holy Toledo! I love them. I use regular old strings on my other solid body. But when you get up the neck on the fretless - it sounds fantastic with the flatwounds.
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