Well I find typing on the keyboard faster on the iphone and also the autocorrect seems better to me, I make loads of mistakes on the sgs3, also I find management of emails better on the iphone as I have 5 email addresses and one combined inbox including GMAIL, whereas on the SGS 3 gmail is a separate app then the other email progarm for the others. which means I constantly have to go back and forth between them.
Also the browser on the iphone wraps the text nicely around so that when you double tap you can read each column easily without having to scroll. I do realise that you can get this if you install chrome beta, which I did, but then I noticed rendering issues when you scroll around the page sometimes.
I think because the 4s is smaller it's easier to operate with one hand.
But I am not knocking the S3 though as it is a killer phone, and I love the screen contrast with the blacks, but I think for me I prefer iOs. (will probably get burned for saying this)
Also I find the Music player in the iphone much better/louder than the S3's one. Plus I do have itunes match as well which means my whole collection is accessible even when I'm out and about (if i have a data connection that is).