They're both great phones. You can read lots of bumph and fanboy rants but IMO it comes down to a choice of Apple or Android - do you have a preference?
If not, if you use many Google services, have a Gmail account, etc. I'd go for the Galaxy S3 (I have a Galaxy S and will probably upgrade to the S3 later this year). If you're not particularly technical, if you have other Apple products such as an iPad or an iTunes account, I'd go for the iPhone 4S (my 15 year old son has a 4S and loves it) ...