I'm planning to get an external HD from LaCie. I'm currently considering, LaCie Mobile Hard Drive, Design by F.A. Porsche (USB2.0) 40GB 5400RPM 50GB 4200RPM Both for the same price. What's RPM and will it make any difference at all? Plus, i'm considering this one as well, LaCie Hard Drive, Design by F.A. Porsche (USB2.0) 80GB for the same price as well. The problem is, the Mobile HD doesn't need an external power and this one needs one. Which one should i really get?