I can tell the difference between 30 and 60. Never tried 120 or 144 Hz. I cared more about the image quality when I bought my first of these two monitors five years ago. I can easily tell the difference between 1920x1080 and my native 2560x1440. Can also tell the difference between 2560x1440 and my TV's native 3840x1440. For other graphics settings, it varies.
But, I also just don't really care that much. The time between generations bores me, because it's when gamers everywhere expose themselves as such graphics whores. Most of my favorite games look pretty crude compared to what's coming out now, but it doesn't bother me at all. I'm also more engrossed right now in Valiant Hearts than in any AAA game I've played in a long time, and preferred most of my Switch games over the more powerful PS4. (Favorite PS4 games were Gravity Rush Remastered and Shadow of the Colossus.) Graphics have little to do with my fun, and they often bloat production costs to the point that the developers have to make their games more for everyone in order to recover costs. I'm saying prettier games are usually more by the numbers and safe. Right now, I have so little interest in the next gen graphics cards. Will stick with my GTX 1070 for probably quite a while longer. I don't think there are any killer apps for the RTX 3000, meaning games I'd wanna upgrade for.
How many people even watch TV anymore? I haven't watched TV in about thirteen years, and I know I'm odd, but even observing other people, it seems most just consume everything on Netflix, Hulu, Disney Plus, etc.
I buy 4K movies on disc. My collection is at 51 titles now. I would say nearly all of them are huge improvements over the 1080p YCbCr versions, thanks to much higher bitrates, more even grain structures, obviously better defined details and colors that more accurately match film. I can barely tolerate streaming quality. I also like having my TV for unmentionable photosets, most of which have even higher resolutions.