Which should I get, Rainbow six vegas two, or army of two? I've heard army of two is really short, yet I've also heard that it's better than Rainbow Six.

Army of 2 aswell as being fun is still not a strong buy in my opinion, I reccomend you buy it when its cheaper. But go with RSV2 you will have alot of fun with it.

Yeah get RSV2 its better than Ao2 anyway.

Its not fun if you suck at R6V2 because you'll get owned unless your good at the start or have played the first vegas. Ao2 is not an extremely good game in terms of reviews but it is really fun but short in story mode. I dont think anyone plays Ao2 online so go with R6V2 unless your a noob gamer.

well, the thing is, I am a live player, yet i feel story mode is the deciding factor, which is better?

Well nether really have an outstanding storyline. If your really intrested in live, rent obth and see which you like best.