I actually replaced my iPhone 4 with a GS3. That's lasted just over a month until I got the iPhone 5. I can't say anything bad about the Samsung but after using the iPhone for several years I got used to it and the way it works.
The GS3 has a nice large screen but that's about the only plus that I liked. The email isn't as good, including having to download attachments and pictures to the gallery to be viewed. Also the size of the GS3 does make it harder to use with one hand.
It all comes down to what you want and what your use to. For me I like the iPhone although at times I miss the larger screen of the Samsung.