Now, this could be completely wrong, economically... I have no idea, I'm not a business major. But:
I chose disk. Mainly because I don't know how moving all of our stuff over to USB is gonna affect other prices, and how they will even affect themselves. Basically, you're taking away the fixed cost for the disk drive, but you're moving over to another medium that relies on parts that are used in massive quantities in other locations.
Sure, right now it's fine because most people buy a USB drive and use it for a good while. Even if everyone in the US has one, it's not something that you necessarily need an overwhelming amount of, because at most everyone only really needs one.
If we move video games over to USB drives, I think it's going to slowly (or quickly) price hike stuff that relies on similar parts to it... which in turn will make the USB drives more expensive, because it's a vicious cycle.
Could be wrong... I don't know how much supply there is for USB parts, or what they're used for elsewhere.