Check out the game report screenshots to get an idea of how FOV works with wider aspect ratios--for games with Hor+ behaviour the centre display on a triple-wide setup often looks just like a single display apart from some HUD position changes. The skewed perspective of the side displays is obvious when looking directly at them, but focus is primarily on the centre display (this is also why the ideal is to have the multi-monitor HUD restricted to the central display). 21:9 is much narrower so its edges have comparatively less distortion.
Game output is a flat image. There is no capacity for correcting the output for curved/angled displays. Display curving is subtle (e.g. see skipclarke's
LG 34UC97 Curved 34" 3440x1440 Ultra-Wide Display review and
video follow-up). The angle change between displays in a multi-monitor setup is noticeable when looking directly at it but you're not going to be doing that during normal gameplay.
How much the bezels bother you will come down to personal preference (of course narrower is better). 21:9 has the advantage of a much wider view before any disruption. Mixing displays (central 21:9 with 16:9 sides) is another option here but note that this is not supported by Nvidia Surround (AMD Eyefinity supports it). Gaming with multiple 21:9 displays is possible (e.g.
LinusTechTips experiment), but as Linus notes you will deal with even worse HUD placement in games with no HUD correction and some visual errors towards the sides in some games due to the extreme FOV involved.
As for performance and hardware advice I'll leave that to someone with more experience in that area.