Plastic. I always thought the aluminum looked kind of tacky. Especially since there was black plastic at the bottom. Most had a case for it so you really didn't see it anyway.
With a plastic back, it's actually a little lighter even though there's more stuff crammed in there. Like others have said it doesn't block reception as much and the plastic feels really great in your hand.
I'm sure the aluminum is better with drops though. There was a guy in the iPhone 3G line who could actually slam down his v1 and it's still okay. I hate when he does that.
