Well mate that's a hard choice you have there, i have both phone so i can tell you that all depends on what you're looking to do or style.. 1. The only reason i would pick an iPhone is because when you jailbreak it, you get EVERYTHING Free but beside that the S3 is better in every other way...
2. The s3 have better display
3. S3 is cheaper with a 2 year contract :0
4. Trust me you will be better with a S3 unless you want to Jailbreak the iphone for free games/ apps and game hacks... hope i help mate..
Iphone 5 is a very Good phone and if You like to Face Chat its the Phone for you, But S3 has way better graphics and has less chance of Droping calls then Iphone 5