Depending on what your background is, you can also try Playstation Mobile, which runs on Vita and a few Android phones (will become more). It's currently in open beta and free, and comes with a lot of game related examples. It is a lot like XNA (C# based). Don't know if it is good enough yet for 3D stuff, but it's certainly interesting, and if it takes off, it could make life for Android developers a little easier. I certainly much prefer it simply because I'm not a fan of Objective C.
Right now of the platforms you mention iOS is a better eco-system in that more games are avaible on it and more people buying it, but for that same reason you might as well consider Windows Phone 8 / Surface as your target, as when that launches, there will be less competition commercially than you have on iOS (same for Vita).