Yes, 24 4k is better than 27 4k (other than size), in the sense that it has got more pixels per inch, 185 vs 150 I think. 150 is still good but to get Retina experience you need 180-200 dpi on an external monitor. So to me 4k on 24k and 5k on 27 are the sweet spots.
I use my monitor at 30hz (Lightroom, web browsing, MS office) and is not bad at all, hardly tell the difference. Try 30hz on your current monitor and see if you can live with it.
Damn, wish I hadn't read that.....now you've got me thinking I should get a 27" 5K monitor!
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Then again, that would be dumb because the Mac Mini won't support 5K resolution anyway!