I have used every mobile OS from Palm, WindowsMobile, Windows Smartphone, Windows Phone 7 & 8, iOS, Android from eclair to current, and honestly its the hardware that drives me more than anything else. Every time I thought iOS (or any other OS) for that matter would be better I always find myself returning to Android. Why? The hardware options are better, the OS improves with every iteration and yes we do have to wait but that is what happens with an open system that allow for greater choices.
If you like the iphone you have today, I would say stay with it, if you want the full benefits of Android and what Google can do for you then I would say sell it now while the market is hot and get the GS4. There will always be an even hotter Android phone in some form or function around the corner but honestly the Galaxy S series is the one to watch.