For me, that's an easy one owning an X34, and while I appreciate the extra pixels and sharpness you get with 4K, I wouldn't want to lose the immersion of 21:9. The difference in width is nearly 20cm between the 32" 16:9 and 34" 21:9, so that's quite significant. Of course, it all depends what you value the most... for some people that sharpness is the most important factor. Only you can answer that.
Personally, I would want at least 40" before I jumped to 4K... and to be honest, even though the 1080Ti is a stonking card, 4K still demands a lot from it. At 3440x1440 though, you're going to get much more longevity out of it.