FireWire is DMA based and uses low-latency network stack. USB 2.0 is interrupt driven and hogs up your CPU utilization. USB was designed for low bandwidth Mice and keyboards and was rigged to work faster under 2.0 but it is not an elegant solution by far. Firewire is the gold standard, don't get taken in by the claimed speed ratings, get Firewire whenever you can.

If you are going to have a hard drive in your MP3 player, it better be able to act as a drive as well. Plus the iPod is still more elegant in size and design and works with the Music Store and high quality MPEG4 AAC files.