With Android, the back button's a case of mystery meat navigation: App developers are able to override the button's behavior to implement their own semantics for the "back" command, which means that for the end user its behavior is unpredictable.

The standard on iOS is to implement navigation buttons in software as needed. Done right, this is better UX, because different apps can provide different navigation buttons to support different navigation schemes.

iOS's One Button to Rule them All also behaves different ways in different contexts, but the way it behaves is fairly easy to learn because it's consistently linked to the screen's visual state.

In short, neither is terribly great UI. But I think this is one of those cases where which one is worse is entirely dependent on which one you're more used to.

Android is removing physical back buttons and iPhone's had the single button work well with the interface since 2007...

Coming from a Blackberry Storm, my fiance tried an iPhone 4S and said "This is so complicated. Why is there only one button? What's it do?"

Ended up with a Droid Incredible 2. She loves it.