Having used the S2, and switched BACK to an iPhone 3Gs, I'd say, that unless you're doing heavy web browsing/streaming, and video playing, the iPhone all the way, IMO.
Found the S2 was good for a lot of things, but the main things (widgets, and faster internet, etc) weren't a Plus enough to use over an iPhone.
Not sure what the S3 brings, but I personally, am looking at upgrading to the iPhone 4s possibly, rather than the 5. Although, I've just heard the 4s has a glass back on the phone, which is a bit silly, IMO.
Having used Android since I had the HTC Desire, I don't see myself switching back to Android for a while. iOS is great for me, and my needs.