I sold my S3 to buy an Iphone 5 as well. Very happy with it! I love the build quiality and the fact that I can operate the phone with just one hand. Sure the S3 screen is great when at home sitting on your couch or something, but on the move the Iphone is more handy to me.
4s is also great but the 5 is really fast, will hold it's value better, if you can miss the 199 pounds...go for it. If you can use the 199 pounds really good for something else, the 4s is still a great phone. So get the 5 unless you have a really good destination for your 199 quid now.