It really comes down to personal preference, and what works best for you. The iPhone 5 is a fantastic device and would be a massive upgrade from your 3GS. Rumors are floating that the 5S will come out soon - perhaps as early as June/July - but September/October at the very latest. If you can hang on a couple of extra months you'll get the bigger screen of the i5 and latest iOS, but with some improved hardware.
Without playing favorites or admitting bias, the best thing you can do is get your hands on a phone running the latest version of Android Jelly Bean and see how you like it. If it suits you better than iOS, then look at the hardware: compare the GS4 and HTC One, as they seem to be the best Android phones out there. If you're a fan of iOS then don't waste your time, just figure out how long you can wait, then get either the 5 or 5S.