I will have to say my opinion is biased as I have the iPhone4 and really enjoy it.
For simplicity and yet with enough variety the iPhone4 is second to none and there is just about an app for anything.
As for the Samsung it is based on the Android platform and the only complaint I would have is the crapware the phone carriers are allowed to put on them e.g. rogers store or whatever etc.. (Apple does not allow that)
Other than that either is just a matter of getting used to so I hear (with Android)