Plastic may be stronger or weaker, whatever, the point is that it's transparent to radio waves and it allows the whole back to be one piece, which looks nice. However, it needs to be thicker than metal in order to hold, so the entire device will be thicker. It's not painted at the surface, the paint is mixed into the plastic, so scratches don't show up a totally different color than the paint. Plastic can be made strong enough to not crack, and if it's good quality, it will be more elastic and therefore won't dent or break, as opposed to metal or glass. It feels a bit less professional in your hand, and looks a bit cheaper too. I guess those are the pros and cons! I don't mind either way, I'm sure it will look nice.