And thnx again Havocsarmy I would have bought one of these games at full price if it wasn't for you bro.
I've no idea about UMvsC3 but I do know it's limited in how much single player content there is~.
I haven't tried it, but I heard a lot of people were disappointed with SF X Tekken. I suppose that could be the case with UMvC3 also, but I heard more positive about it. It looks like both were reviewed very similary though. I have UMvC3 and really like it; if you like the other MvC games, it should be pretty similar. The only disappointing thing for me is that it doesn't have as many as/certain characters that were in the previous ones.
I haven't played SFxT, but lked UMvC3 enough on my 360 to make it my first owned game on the Vita.
Other then that, here are a couple videos done by the same guy on both games on the Vita. Maybe it will give you a better idea of how they play and you can see which is more appealing to you.