Matte = great
I have an iphone 2g in a clip-on case made of this kind of rubberised plastic. It's great - after months of heavy use, the only marks it shows are a little polishing around the corners. The aluminium under it would have the same polishing plus scratches, and gloss plastic would have scratches, so I reckon it's tougher that either iphone.
The main thing though is the feel - without the case, i'm forever dropping the phone because it's so slippy. The 3G is better, but not much. But with a rubberised case, it almost sticks to your hand. And no, it doesn't stick to your trouser pockets. It's not 'sticky' or anything, just not at all slippy.