for the money, if you want to save a few bucks vs buying a premium game, i would go IM pro. and i love the other 2 games. the music of IM can grow on you. as you progress, the light show can be really nice. also, the game is deeper than you think.
as far as difficulty, i feel batman is probably the most challenging of the 3 games. star wars is probably the deepest game. i feel it gets ripped by those that have not played it enough. there are a lot of missions in it. great light show too. i think the pro is better than the premium there.