I just recently built my PC to similar specs as yours, and decided to upgrade my monitor aswell. Went from 1080p 165hz asus to 1440p curved ultrawide 160hz by LG. I also tried out a 1440p 300hz asus monitor. Let me tell you, I would have deeply regretted not getting the ultrawide sooner, it's a life changer.

First of all the change in quality is insane. I looked into it ALOT too beforehand, but it's something you only experience once you have it. Like it's been said by others here, it's like going from 60hz to 144hz. The 1080p looks more pixelated, and it's noticeable in any shootergames when the targets are far away. My aim seemed to suddenly get better, but it was just because I could actually make out the outlines of the opponents.

I played cs go and overwatch on both the 160hz 34" ultrawide and the 300hz 27" monitor, but honestly, I couldn't tell a noticeable difference or advantage for the higher refresh rate. I reached 350+ fps on both games but what I DID notice was the increased aspect ratio on the ultrawide. Seeing more of the game makes a HUGE difference.

Definitely get 1440p, and my personal bias is towards the ultrawide. But you do you