In recent years I've used a 27" 1440p 144 Hz, a 34" 1440 ultrawide 165 Hz and currently use a 42" C2 OLED. I've also used a friend's 32" 1440p 165 Hz and another friend's Odyssey Neo G9.
The Neo G9 is cool for gaming as you get a lot of peripheral vision, but after gaming on my C2 for a few months I really missed the extra height. The G9 sucks if you plan to watch a lot of films or YouTube on it. It just becomes a 27" monitor with big black bars either side. It's also not the easiest thing to use for web browsing as you'll end up turning your head a lot unless you centre the web browser.
I think the 42" C2 is a better size than the 55" if you're going to use it on a desk. The 49" G9 and 55" TV may sound close in size on paper but the TV will dwarf it in reality and be too big for all but the biggest desks. I have my C2 maybe 70-80cm from my eyes and it's perfect for general gaming, films, web browsing, etc.
The G9 is slightly easier to run than 4K but not by much. It's something like 7.5 million pixels versus just over 8 million for 4K.
The sweet spot for being reasonable to run, a good size and aspect ratio for everything, reasonably priced, etc. is probably a 32" 1440p monitor. I was expecting it to look horrible with the same PPI as a 1080p panel but it was fine.
TL;DR: a 42" C2 if you're going to use it on a desk and the 55" if you're using it in a living room or similar setup. I'd only really consider the G9 if all you ever do on it is game, but I found myself wanting the extra height of my C2 back.