Standard firewire is rated at 400mbps transfer rate and USB 2.0 is rated at 480mbps. However USB 2.0 is not as efficient, so real data transfer rates are slightly higher on firewire. As already mentioned, firewire also has some additional features like daisy chaining and device-to-device direct transfer. There’s also firewire-b which is at 800mbps, and there’s a firewire-c standard coming out at 1600mbps. However firewire-b is not yet widely supported, so the number of devices are very few. USB 2.0 is cheaper to implement, so devices will cost less, but Macs and MiniDV camcorders still use firewire almost exclusively.
Practically speaking though, there’s not enough of a difference between firewire and usb 2.0 to worry about. Therefore you should look at what you will be using it for. If you have a Mac, you will likely want Firewire. If you want to play with Digital Video cameras, you really want to use firewire. If you have a PC and want to connect up disks, digital still cameras and scanners, usb 2.0 is the best choice. It all depends on what you want to use it for.