I'm not sure that comic book power levels are all that relevant. There is no Galactus in the MCU, and you can see on the gauntlet, all the gems are in one place, so it's not like there's some separate storyline for the time gem or the power gem or whatever. Also, omnipotent villains in movies are cheesy because they have to do ridiculously stupid things to lose. Expect to see Thanos toned down so that a plausible (in terms of an intergalactic conqueror being smarter than an average child) story can be told.
I still don't quite see Ultron. Especially since Edgar Wright's Ant Man is going to be set in the 60s, if it even comes out before Avengers 2. Maybe Pym can have invented Ultron, but he won't be on the team. Would people still want Ultron if the Pyms aren't in the movie?
Kang doesn't have any such impediments.
If anything, he can actually bring 60s Pym into the present day on a temporary or permanent basis. Ultron can't transport the Pyms through time to be in the Avengers movie.