i have an external usb2 maxtor hdd, and it works great, almost as fast as an internal. I think although fiwi is supposed to be a bit faster, at the end of the day it won't make a real difference to you. so, i would suggest go with the cheaper price, both seems to be working well.
firewire provides the same or more real bandwidth than usb2.0 because it has less overhead since the interface isn't as universal as usb (universal serial bus). on top of that, it also uses less cpu time. go firewire for external drives when you have the option.