Both? Completely depends on use case. I have an iPhone with ProMotion and I have two Macs that don't. I never look at the Macs and find the movement of things on screen jarring and rough. I also don't do any real gaming. So, 60 hz is absolutely fine for my purposes.So, what's better, 4K at 120-Hz or 5K at 60-Hz and another hundred bucks?
On the other hand, what I DO a fair bit of is working in graphic design, in which case perfect pixel scaling and high PPI is a big help. The bright 4.5K display on my M4 iMac at home means I can see fine line details accurately in Illustrator without constantly zooming in and out. That's a 24" display, but if I wanted to spread out and maintain the same PPI I'd surely be looking at a 5K display @ 27", like the Apple Studio Display or similar -- because it's well suited to that kind of work.
So, for me, 4K at 27" is I'm sure OK but not optimal, but 5K at 27" is a sweet spot for both pixel accuracy and PPI, and refresh rate is not a factor. If I was doing totally different stuff, needed HDR for video or did a bunch of gaming or whatever, then yeah, maybe a 4K 120hz display would be my pick.
I feel like so many heated arguments on here can be avoided by just remembering that different people have different uses and preferences and that's normal.