What scales better at 4K? 27" or 32"

The larger of the two. From what I understand, 40"-43" is the sweet spot for rending 4k without scaling. I have a 40" 4k that I plug my mbp into and it is superb.

 

The bigger the better if you want to run at native 4k and still be able to read it. If you want to scale it(ie as looks like 1440p by default) then it will make no difference to you.

 

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).

 

It depends on what scaling you want to use. Pixel doubling is the clearest, which would be 1080p. Some people need that at 27" or even 32" for their eyesight, while some others just prefer it.

Intermediate scaling has little difference. More important is would you be using the extra inches for more workspace? Or having things bigger?

 

I have a 27" retina iMac as my main screen. It is 5K which gives 1440p as a perfect pixel double, and gives a great text size. My external monitor is a 27" QHD, so is also 1440p. The text is the same size, but clarity is far weaker.

If I scale my iMac, the clarity is not quite as good, but far nearer perfect pixel double than the external monitor.

You should be fine with the 27" for clarity, unless you are doing something like image editing and you want to see pixel-by-pixel, but you could always change the scale for that task if it is occasional.

 

One of the few times Apple didn't "gimp" something artificially.
;-)

The chipset as provided by Intel could always do 4k@60Hz. But IIRC, it only worked after an OS update, not from the beginning.

Probably helped by the fact that Apple also sells a 4k display....