15" with a Western Digital Scorpio Black 750GB HDD. Good speed at 7200RPM and high amount of space for your media needs. Go for the low end model and use the case you save to get the HD from newegg or somewhere.
I prefer Western Digitals, but we use Seagates at work and those are fine too. I think if you stick to those brands you'll be fine. Buy USB3 if you can, because...why not? It'd be a few bucks extra for the new technology, especially if you carry around to PCs and such.