None of them to be honest. 34" is too large to have as a completely flat screen, the viewing angle issues at the extreme edges are enough to ruin the experience. Getting a curved one helps, but even then a lot of the curves are rubbish and effectively flat anyway.
The 1080p Z35 is long in the tooth and hideously overpriced whilst the replacement has been delayed.
The issue with 1440p ones is that even if you are okay with the colourshift/glow at the edges, the resolution is too demanding to run at decent refresh rates, you'd struggle to break 60fps in a lot of modern games at that resolution unless you are running a GTX1080 or Titan X(P).
Basically, ultrawide 1440p is almost as demanding as 4K and even if you decided you want one, of the ones on the market, most users are complaining of manufacturing issues. The LG, the Asus and the Acer models all seem to have some rather serious defects in terms of panel uniformity and backlighting. Given their exhorbitant cost compared to 60Hz 4K monitors, I'd be boycotting them until they do the job properly and for a sensible price.
Both of them are curved monitors. I didn't notice any shift or color change in them and no light bleed (bright showroom though). I was really taken with these monitors, the curve seems to draw you in whereas the flat 34 inch screens don't. I didn't really look at the 1080p ultra wide monitors but I did look at the ROG Swift 1440p IPS and the TN screens that were sitting side by side. I don't think the difference between these screens is very much at all. The colors were a bit better on the IPS and you really had to get off to the side of the TN to notice color loss. For gaming I would pick the TN as you just don't notice the color difference when you are doing an FPS. I would love to have the 34 inch ultra wide ROG for playing Elite Dangerous tough. However, my Tobii Eyes won't work with that screen size and my GTX 970 won't drive it hard enough.