Hi there,
I'm deciding between the LG 27UK850 (Which I pre-ordered already) and the LG 32UD99 which i've been eyeing up.
I'd never use either monitor at full 4K resolution, but in your experience, what does everybody think scales 4K better on Mac OS X, 27" at 3840x2160 or 31.5" at 3840x2160.
I don't know what's meant here by "rendering without scaling". A 4K monitor that's 27" has the same number of pixels as a 4K monitor that's 32" -- those pixels are just spread out over more space (and therefore less sharp) on the larger screen, which may or may not be visible depending on how far you're using it from your face.
If you're talking about the scaling of the UI in macOS, that always gets scaled up on a retina screen, because if it didn't your UI elements would all be very very tiny.
For reference here, the iMac 5K has 5 built-in "looks like" scaling levels available -- these basically tell your machine to emulate the UI scaling of a non-retina montor of the following sizes. It's sort of a kludgy and inexact way to refer to things, but it gives you a basic frame of reference.
1600 x 900
2048 x 1152
2580 x 1440 (default)
2880 x 1620
3200 x 1800
For the 27" screen I have, I find that the default looks good to my eyes, about 2' from the screen. But that's quite subjective. I'd assume that if you're using that big 31" screen you'd pick one of the higher ones (with smaller UI elements).