I have an iPhone 4S and DH has an S3. I love my iPhone-I've had it for almost two years and it runs like the day I bought it (and I'm about as hard on electronic devices as you can be-I managed to crack an OtterBox). DH has had his S3 for about 10 months and despises it. It was great for the first couple of months (maybe even superior to the iPhone) but then he started having issues such as the phone freezing and the touch screen not responding. He is begging me to get my upgrade so that he can have my old phone, and he is a guy who always has the most up to date electronics.
People can argue both ways, but if battery life is important to you, definitely go for the iPhone. The S3's battery life can't touch the iPhone. I can get 2-3 days out of my phone using Bluetooth, GPS, and email going at all times. My husband does well to get a full day with low to moderate usage.