I went from an iPod Touch to a Motorola Droid—not exactly what you're asking for, but I'm at least familiar with both iPhone OS and Android.
Like the others have said, you have a ton more customisation options with Android, even unrooted. Don't like the default keyboard? Just plug in another one! Custom ringtones are easy to make (even for SMS notifications), and can be done right from the phone. In fact, almost everything—installing new software, installing system updates, and even rooting and installing ROMs can be done right on the phone.
Also, the power of Google's cloud sync services is excellent, and the Android notification system blows the iPhone's out of the water. The point about push notifications is a good one, but at least those notifications sit patiently in the tray rather than jumping into whatever you're doing at the time.
As for iPhone advantages, I understand that it has much better support for Exchange, so that might be something to take into account if you need Exchange. And if you're the kind of person who has major affection for the polish of Apple's UIs, you might find Android to be a bit clunky at first. (But once you get used to it, you really appreciate its power.) I have to say that there are only two things I even somewhat miss: the game library and the software keyboard. And since discovering Swype, the latter has been less of an issue.
Oh, and the iPhone media player is better than most of the options for Android, but I kept my iPod Classic for my major listening needs, so it doesn't bother me
Hope this is helpful—if you have any specific questions, just ask away!
