to tell you the truth, they are both amazing phones! but the Galaxy S III offers a lot more customization options then the Iphone 5. For example, on the Galaxy you can ove widgets around and customize it to the last detail. Anther great thing with the galaxy is that most apps are free, and if they cost money, you can just download a different app store where it would be free! Imagine the Galaxy S III as a jailbroken iPhone without the lags, crashes and inconvenient things that come with jailbreakking.
Samsung obviously has a lot more feature than iPhone. And iPhone 5 is not really that different from 4s. It just has a longer screen and better graphics maybe? So i think getting an iPhone 5 would be a waste.