You have to do what is right for you. I spent a year on Android and I liked it but I opted to return to iOS.
I'm in the same boat. Got the iPhone 3G when it first came out, and left the iEcosystem for the HTC Evo when it launched. I've since upgraded to an Epic (since my Evo broke). While there are many things I love (and will certainly miss) about Android, there are just too many things that don't quite work.
So, iPhone 4S it is! This time I think I'll stick with Apple though, since I certainly don't have any desire to try a BlackBerry, and WP7, though cool looking, doesn't seem like an OS I'd want as my daily driver.