it depends on your application of said hard drive. If you're simply looking for back up room, then go usb, it's cheaper. If you're looking for an addon, go firewire, it's faster. Keep in mind, however, that it is possible to upgrade your internal hard drive. while such a procdedure will void your warranty, it is ultimately worth it. you can then take the older hard drive out and put it in an usb or firewire enclosure.