How would you compare the gaming experience at 120hz vs the 60Hz 4k?
Well the 120hz is a TN panel so colours are worse, view angles are worse but typically not much of a problem from a stationary position. The size due to small resolution of the TN is a minus, the 4k is 32" which is a really nice size for gaming at a desk on. The 4k despite it's much bigger physical size still has a PPI advantage just due to the sheer increase in resolution.
The TN has less ghosting but I don't typically see it on the 4ms IPS, I did on the 8ms old one, but really it's hard to notice on the 4ms panel, even during "obvious" examples, such as high contrast, black on white with high movement. Of course 120hz is nice to have, you need to maintain a very high frame rate, and FPS is a non linear measurement of performance so going from 60 to 120hz really takes a lot of GPU horsepower, I think it's a really great way to play competative and semi-competative online gaming, I think that's where >60hz can be justified, especially something FPS and especailly more twitchy games like arena shooters, for me personally games like TF2 at high frame rates looked really smooth.
The 4k wins hands down though it just beats the TN in too many categories and where it fails it's really not that bad, 60hz is certainly very good for most single player games and acceptable for anything non-pro level in competitive online gaming. And certainly with 4k 120hz isn't an option anyway even if there was panels that supported it, it's simply too much horsepower needed to give a smooth 120hz experience at that resolution, so it's not as if anyone with a 4k monitor is super worried about getting a 120hz upgrade because we just do not have the GPUs for that right now. We will eventually though.
A large 30"+ 4k IPS is such a good investment right now, the prices are great, my last IPS which was the 30" dell 2560x1600 lasted 8-9 years as my primary monitor, well taken care of, that's a really good investment over time, and I feel exactly the same will be true of this one.