How much should I pay attention to pixel density? I read somewhere that Mac OS looks best on 110 or 220PPI.
A QHD 27” monitor seems to be closest to this pixel density.
I’m currently deciding between the ASUS PA278QV (QHD), Dell U25220D (QHD, slightly smaller screen) and the Dell S2721QS (4K).
4K on 27” should be nice and sharp, but the PPI is 176. Should I be worried about this? Is scaling likely to be an issue?
It depends a lot on how you use it, and how good your eyesight is.
~110 (for 1x, or non hi-dpi mode) and ~220 (for 2x, or hi-dpi mode) are generally what Apple displays use. So the 5K iMac/5K UltraFine display are 27", and display a picture that
looks like 2560x1440. The pre-5K 27" iMac and 27" Thunderbolt displays, were 2560x1440 native res.
There are numerous 2560x1440 displays on the market if that's what you want.
If you want a hi-dpi display, that means you're generally looking at either a 4K or 5K display. There are heaps of the former on the market, and only generally one of the latter. A lot of people end up with a 27" or larger display, at 4K. For some this will be fine, however by default in macOS it's going to
look like 1920x1080. You can change the resolution it "looks like", but this will mean slightly less 'pixel perfect' display (likely only important if you're doing graphic design) and also means there's more load put on the GPU.
In my experience a 2018 Mac mini's iGPU can do non-default scaling for
one 4k display without issue, if you have enough RAM. With two displays it will not be great. Turning off transparency in the UI helps a lot, but it's still not perfect.
If you can afford one, I'd suggest the LG UltraFine displays that Apple sell (either 5K 27" or 4K 24"), simply because they're the closest to "ideal" PPI for macOS.