I am currently making the same decision.
I have a verizon i4, and just got a Sprint GSII. I have 14 days to decide to keep the GSII, or exchange it for a 4S.
I do love that big 4.5" screen. It's great for tv shows, and to browse the web. Gaming also rocks on it.
My favorite feature of the Android OS is that apps can access data in the background. It can download weather updates, IMs, feeds, etc.
The iPhone has push, but when you tap the icon it still has to download all the data.
The iPhone however, really has the most support. There is tons of games and apps available. The airplay mirroring is also gonna be great. I can pick a show on air video or a game and just transfer it to my TV wirelessly, now that's amazing!
I really have to try iOS5 before I make a decision. It's gonna be a tough choice.