I've got an S2 my dad's got an iPhone 4s and is switching for android.
The iPhone is all linked together very well but I think they have over looked some very basic things like auto send text when you haven't got any signal for instance.
The iPhone's screen doesn't seem to get as many finger prints on it as mine (if that is relevant even slightly to you)
The other thing to consider is battery life, although the iPhone isn't a bad battery and lasts as long as mine you cant extend it without buying a power pack etc. also to change the battery it has to be shipped off. the other thing that would worry me is if it froze you can't rip the battery out.
We are both on orange but I usually have better signal when we are in the same place, also he has been connected to a random persons conversation on 3 ocations when placing calls, which is great if your nosy
iPhone has no signal boost so no phone calls in the house if you have no signal, just internet browsing over wifi.
I didn't really understand what people meant by the iPhone is smoother until I used my dad's for a few days, it is hard to explain but it just is.
I personally didn't like looking around to find how to go back to the previous thing you were looking at on the iPhone, it's really nice knowing where your back button is without looking for it.
Also I don't know about anyone else but i like having a picture as my home screen rather than a load of apps.
Wow didn't realise I how much I didn't like about it in only 3 days of use. Don't let this put you off though as I have had to put a bit of thought into this and was just trying to be a detailed as poss. plus there seems to be a lot of prestige with owning an iPhone.