90Hz is definitely a nice improvement. I don't see any additional benefit between 90Hz to 120Hz, so I would call 120Hz a marketing gimmick. It's just a way for manufacturers to gain bragging rights and charge you more for a display that drains the battery faster while giving you functionally identical results.
On the other hand, I won't go out of my way to get a 90Hz+ capable phone. My last one could do 60/90/120, but my current one is only 60Hz. The visual aesthetic improvement of a higher refresh is only a small item on the list of things to compare.