I have a SGS2. I had the best of the iPhone 3GS models. I've played with an iPhone 4S. Have NOT messed with an SGS3.
The SGS2 has more features and more choices. It has more storage if you put in a card. It escapes the AT&T jail, and whatever other "full-featured" carrier you might be stuck with if you get an iPhone. The apps seem to be more capable IMO, because they're not limited by Apple's insistence on not competing with companies which contract with them.
The SGS2 is big. I have large hands, and I find it difficult to operate the phone with 1 hand. You might want to think about that just a bit.
The iPhones are definitely more polished. In the 2 years of using an iPhone, I never once had to ask how to do something that the iPhone actually allowed you to do. You just think what you want to do, and go where you would think that thing would be configured, and then use it. The interface is more consistent across all the apps and all the built-in features. The phone itself and IOS has had a single, extremely skilled team of designers who have guided the entire development in a specific direction.
The Android stuff on the other hand has less direction and more flexibility. The first day I had it, I had to use Google to figure out how to get my contacts on it, and it was still a PITA.
On top of all that, the same arrogant commercialism that made me not like Apple very much is still there with Android. Your carrier will almost certainly be more interested in profit than in service, and Google wants to collect statistics on everything you do, and store every email address and web site, and manage everything for you. So on that side, Android sucks as much as Apple.
All in all, I prefer the Android because of the more sophisticated apps, but maybe next time I'll pick one I can run with one hand comfortably.
I seriously think the SGS2 is a better phone than the iPhone 4s. An SGS3 has got to be better yet. But everything I used to judge is subjective, and somebody else might have a different opinion about the same exact characteristics.
Good luck and have fun.