21:9 support is definitely improving in games, but there are plenty of games (especially older ones) that dont support it, which means playing with black bars at either end.
If the answers to following questions are yes, then I'd suggest going with a 21:9 monitor:
1) Do you only play modern games?
2) Do you use your system for work?
3) Do you like to have multiple windows open at once?
4) Would you be ok with running some apps/games with black bars?
5) Do you have the GPU horsepower to run a 21:9 resolution?
If the answers are mostly no, then go with good old 16:9.