I had an iPhone 4 and upgraded to a 4s, simply because the wife needed a new phone and was happy to take my 4.
I'd say the iPhone 4 is honestly 90% of the 4s, Siri is excellent for some things and useless at others although I was impressed it understood my accent, or as some of my co workers asked, "Does is spake Norn Iron??"
The camera is noticeably better and i like its UI, it suits me as I'm a complete spanner when it comes to techy stuff. It's convenient and the service i've had from Apple in Belfast has been flawless, exceeding expectations on both occasions. Your mileage may vary obviously. Apple vs Android is getting old now, the arguments are more and more petty every time to be honest, I have no experience with Android except what i've seen on co workers units and it's all nice and lovely and all that
I've been assured though that jailbreaking the iphone is like Neo coming out of the matrix and realising for the first time all the cool sh*t he could do, so i may try that sometime!