I think that if you're leaning towards the iPhone, you should wait it out until the next generation one comes out which is rumored to be very soon, like within a few weeks or a month or so. The iPhone 4 doesn't have quite the specs as the Galaxy S II, so if you're leaning towards an iP4 now, you'll definitely wanna pick up the next gen. iPhone instead of either of these two.
Android cannot use iPhone apps and vice versa.