I had an S3 one week ago. Now I have iPhone 5. S3 has better functionality than iPhone but iPhone is faster. Mainly I prefer iPhone but by the way S3 isn't the worst. So if you prefer functionality you should get Samsung. Now when I have iPhone I won't change it for a S3.
S3, because you are not bound to Itunes, etc... And another point is the fact that you can connect with other phones. not only iphone to iphone... you are more independent if you will buy S3
you could take a look on apple.com there you will see the basic functions. After this there are some more points ... Widgets, connectible to nearly ALL other OS and devices freedom of appstores Slot for SD Cards ability to change the battery "One Click Jailbreak insided" ... so now you know what the S3 Android is.