Which one is more noob friendly?
MvC3 easy. This isn’t close
Which one is more balanced?
More play value/play time?
**Depends. KOF’s Missions will take you much longer to complete and is has story mode, color editor (sounds stupid but its addicting), and other various modes. **
MvC3 has much easier missions and a pretty bare boned 1p. Online netcode is better than KOF plus is has spectator mode for lobbies so it is much more enjoyable with a bigger group online.
Very subective cant answer this. I will say KOF is a much more serious game.
Bigger player base?/Longer lasting one?
Also, is UMvC3 the last one or is capcom planning to release another update ala SSF4 AE?
No one knows but safe assumption is that there will be more characters/stages/etc. eventually.
As for recommending one I will say if you are a casual/casual-competitive player MvC3 is probably your better option (especially if you are only going to play online). KOF is a much better competetive fighter but it might turn you off if you are a casual player due to the poor online play and the steep learning curve of the game.