I went back and forth between an S3 and the iPhone5 when I was looking to replace my 3GS. I ended up going with another iPhone because I liked how it felt in my hand, the bigger screen, high-res camera and the updates to the iOs.
I almost went with the Samsung, but it was just too big for what I needed. I'm not on my phone that much other than a few phone calls, texting and checking personal emails. If I was going to be on it constantly, I probably would have gone with something bigger, but I'm very happy with my 5 and have zero regrets.