Firewire is faster than standard USB, no doubt (regular USB -1.x- is too slow for hard disks, and DVD playback, IMHO)
In theory, USB 2.0 can get up to 480 Mbps, whereas most FW systems are limited to 400 Mbps... In practice, FW performs better and relies less on your CPU. I think that FW is overall a better solution, but USB2.0 products might become more common, and be cheaper.
Some motherboards have built in USB2.0 or FW connectors, if not, you'll have to add a card (see your manual)
The fastest hard disk interface commonly available (SATA 150) can transfer data at up to 1200 Mbps (150 MBps), so USB2.0 or FW could potentially create a bottleneck with disks that exploit these interfaces at their full potential (DVD burners are way slower than this).