I have found FireWire 400 to be faster and more consistent than USB 2.0 on some devices. Your iPod will most likely have come with a USB lead so I would just use this. It is not worth spending the extra plus the write speed on the iPod isn't that great so the difference will hardly be worth it.
It is also worth pointing out that FireWire will not charge the newer iPods, which I find useful when transferring a lot of music, so stick with USB