Honestly, unless you drop the laptop, or put it on services with lots of hard dust and dirt (grains of sand are your enemy!), you won't scratch it. Your best bet is to get a nice sleeve, like the Incase Slim Sleeve, and use the MBP naked. If you're really worried about scratches, you could use a skin and put your MBP in a sleeve.
From previous experience, I would avoid hard cases. Any dirt that gets in that is quite hard will scratch your laptop. You can prevent this by taking your MBP out of the case and cleaning it regularly, but that doesn't really give you anything over a skin and a sleeve.