I'm fairly sure that results will vary person-to-person. Theoretically, they are both very close speedwise. In realworld performance, the Firewire cable is faster on my mac & works perfectly, while the USB cable is almost as quick, but causes some weird battery problems.
Note that unless you have a 4G then you will need to plug the iPod into the wall to charge it, as the 3G won't charge through USB 2. Also Firewire will charge the iPod abit quicker than USB 2. But make sure you get a full-size (6pin) Firewire port (if you go for that option), as the 4pin version won't charge the 3G or 4G, and you also need an extra adapter to go from 4pin to 6pin.
Also note that PCMCIA slots (where you'll be adding your adapter) do not supply power to those USB or Firewire ports. I doubt the add-on firewire card would work because of this. I'd buy the USB card so that you could provide power via the ipod's power brick at the same time.
When I first got my ipod, i tried to use my USB2.0 ports. I had already about 5,500 songs to transfer (had a Nomad before).
I could only get through about 1,000 songs before it lost the connection. My only solution was to reformat and try again. After about 3 times (all dealing with Apple support), I went to Microcenter. The main tech guy there said that what they are seeing as a big difference between USB2.0 and firewire is in long data transfers. After 30-45 minutes, the USB starts to become unstable. Firewire is very stable compared to USB2.0.
I switched to Firewire and all my problems went away. As for speed, about the same. Tech guy said that firewire was actually a bit slower, but I did not notice any difference (except for the whole working thing).