go firewire!
Get the firewire! Every USB powered hdd drive I've owned always comes with two usb connectors because one isnt enough to fully power the drive, and since my Powerbook only has two USB ports, I lose both of them to one drive and two cables makes in more fiddly.
Firewire can deliver up to 30v (compared to 5.25v on USB) its fully powered... and since Firewire 400 is still faster for data transfer than 480mbps USB2....🙂