I still don't quite follow... I have a MBP as well, and my external USB drive still requires no power cord. Are you just talking about speed here?
"fast transfer" implies speed. A USB 2.0 hard drive is suitable for backup (not so fast, though) and for secondary storage that you don't care about the speed. The firewire drive at 7200 RPM is like having a drive built in to your system - it's that fast. If your use for the external drive is to store and view lots of photos or for video storage and manipulation, then the USB is going to be noticeably slow.
Got it -- we are agreed on that point, then. USB is great for backups, but less than ideal for media that you use actively. I was just confused by your earlier statement that you went with FireWire so you "wouldn't need to plug two things into the wall" to use your laptop. There's no reason to have anything plugged into the wall with portable USB drives...
Thought, I'd just bump this thread instead of starting a new one. Anyone know where I can get a PS3 the cheapest?