Hi
I have tried all three configs (VR, 21:9 and triples). Had 21:9 first, moved to triples, then moved to VR, then moved back to triples. That kinda tells you which one I prefer.
I bought my AOC 21:9, non-GSync (3440x1440) a while back before anything more than 60Hz was available. Wouldn't get anything under 144Hz now and ,for me, GSync is a must.
Pros. Less space required for your rig; easy to mount; generally cheaper than triples; requires less GPU power than triples.
Cons. Albeit more real-estate than a 2560x1440 monitor, it wasn't a big enough difference for me.
I currently use 3 x 27 inch ROG Swift (2560x1440) 144Hz GSync monitors (@7800x1440 - bezel corrected).
Cons are pretty much the opposite of the pros listed above (mounting can be a bit fiddly, but once its done, its done). But above all, triples are way more immersive for me. Having the correct FOV allows you to see so much more and be more aware of your surroundings, especially immediately left and right.
I would recommend triples if you have the funds and the GPU grunt needed.