I had an iPhone 3GS with iOS 4 on it for a while and it was a great phone. Games on it are really its strongest point compared to the Android platform but feature wise Android as an OS blows iOS out of the water. "Speech to text" is flat out amazing and adds so much to what it can do, the map application (go figure) works a bit better/faster and the OS over all is MUCH more customizable. The first time I played with my friends Aria, I was blown away (and he was an iPhone user as well).
Don't knock Android, right now the MOST powerful handsets on the market are all Android (sorry iPhone 4) and the most advanced OS is Android 2.2 (you could even argue 2.1 is a bit more advanced than iOS4 having widgets and what not). Don't be afraid of the unknown if you like the iPhones form factor and you are tired of iOS just not giving you enough depth, go check out the Galaxy S line of phones. Pretty much same CPU w a fast (much faster) GPU than iPhone 4 and then a better screen and all the other Bells and whistles including BlueTooth 3! Apple had a good run and is definitely responsible for getting smart phones where they are today, but they have some work ahead of them if they want to stay in the game long term.