What monitor you end up getting depends on what kind of games you play. If you are just an average gamer, who doesn't stick to one genre, 4k is fine - If you play a lot of FPS, I'd much rather get a 1440p, 144Hz monitor with G-Sync, if your budget stretches to that - If you play a lot of games around the racing genre, or where peripheral vision is important for your gameplay experience, 1440p Ultrawide is the way to go.
If you are after my personal experience: I used a 27" 4k and ended up switching to a 34" curved Ultrawide... Needless to say, I'm never going back to 4k. Using a single 980Ti, the gameplay is much better experience as it's 60% of the pixels to drive, which means you can use more eye candy - And it wraps around your peripheral vision a lot better.
As for Ultrawide curve vs flat, I couldn't imagine using an Ultrawide monitor without the curve, it's more than just a gimmick I promise.