I like the dual 2540x1440? that I use at work. But a 4k just fits more... and thus I use that at home. Thing is, I've found I use it at 150% and so it's not really 4k, I don't think? but I'm content with it all the same.
Thing is, my Linux machine doesn't drive it "really well" with 150% scaling and while I can resize a browser window to get a decent font, the rest of the time it's not that great. So I'm a bit divided on it, might just get a second 1920x1080 for it instead of sharing the 4k. The Linux box is for fun, most of the time the 4k is for work--and wow is it nice, huge workspace.
Anyhow. Partial to Dell as that is what we use at work, but I don't have to pay for them. I bought a Samsung 4k before I read that they are junk, but mine has been ok so far? This Acer that I got at the dump works well and it's probably what I'd buy again, the $90 one from Walmart. Coworker had a curved monitor but I thought it a bit weird and when it was up for grabs, nobody in our group wanted it. In my home office, it's in the basement, so a glossy screen likely wouldn't matter, but matte screen seems to suit me.
Distance probably matters. I sit about 24" from my monitors, maybe a bit less. So 24" monitor is fine. But when using duals the size of the bezel matters, less is more, and these days, if it's not a dual monitor setup I get lost trying to figure out how to do something. I also am starting to prefer to have the monitors the same size, otherwise moving the mouse from one to the other is a bit of a pain (collides with the screen edge).
