The black ones have a really fragile plastic that is pretty much guaranteed to crack over time. The "silver" ones (which, honestly, I think are colored more like duct tape) have fixed this problem.
Trust me: you don't want to get a black case. You may think it looks nicer now, but you won't think so anymore when it has a crack in the right speaker, a battery cover with one of the plastic holding thingies snapped off, and a hole in the back/bottom of the hinge. And you'll like it even less if the damage to the battery cover is enough to prevent you from even using your Pandora, or you get worse cracks than the usual ones. Just let them deliver you a "silver" case and enjoy the fact that your Pandora won't look like it was run over by a truck in a few months.