First thing you should ask yourself is which movie/character you liked more. Which figure do you think is going to look the best alongside the rest of your collection? Is Iron Man going to look too vibrant among your other figures? Will Batman be too dark? Also read other topics to get feedback from other users in case there's any quality issues you might be concerned about. There's nothing worse than spending a lot of money on something that falls apart in a short time.
Don't forget Iron Man and Batman have many different costumes. Are you after the BD and DX specifically or are you just looking to add these 2 characters to your collection? If you don't care what version of the characters then Tony Stark would be a better choice since he's one of a kind while the other two will keep getting new variants in the future.
Whatever you choose just make sure its something you know you're going to be happy with.
