UMvC3 is the easiest fighting game to learn out there, that said its also the hardest one to be consistent at. Its very fast paced, very chaotic, and there's a ton of stuff that in any other game would be considered complete bullshit. The game is just very unforgiving, one mistake will more often than not cost you a character (maybe even two). Its a fun game but be prepared, even top level players can get randomed out.
If you're not used to Street Fighter, Street Fighter x Tekken is going to look extremely intimidating to pick up but if you are its not that bad. It's very slow paced compared to something like Marvel, and atleast right now rounds seem to end in time out more often than not. The Tekken characters can kinda compete but the top tier is mostly composed of Street Fighter characters. In its current state its very glitchy, there are also a lot of easy infinites and its not particularly balanced but there is a patch coming soon that might fix most of that.
It really depends on what you want; if you want a very old school style unforgiving fighter than UMvC3 is the way to go, if you dont go with SFxT.