USB is cheaper however and has simpler and smaller circuitry, allowing for potentially smaller devices. Firewire is not supported by iPods any more, but this should come as no surprise as Apple is simply leveraging the massive PC market to fund their projects.
"Use an extra iPod Dock Connector to FireWire Cable for charging and syncing your iPod over FireWire. Compatible with iPod models with dock connector, except iPod nano."
edit - But...I just noticed that the little icons of the iPods it's compatible with don't show "iPod with video" like it does on the Universal Dock. The plot thickens.