Both formats are amazing in terms of what you get for the money compared to what was available to us just a few years ago. As things stand right now the Firewire protocol is capable of passing more data in a given period of time, so for the moment it tends to be the direction people are going for bigger systems (meaning many channels of audio).
The driving force up until very recently has been the fact that USB comes standard on PC type computers and both Firewire and USB come standard on Macintosh computers. However, reliable USB audio drivers have not been available for the Mac until very recently. Consequently PC users have chosen mostly USB systems while Mac users have leaned towards Firewire systems. Now that USB audio is happening on the Mac we are starting to see some smaller audio systems utilize this format. USB allows a manufacturer to build one piece of hardware that can be compatible with both platforms. Obviously PC users can and do install Firewire cards to utilize those systems, but this isn’t quite as simple as just plugging something in. Further, if a card is to be installed there are many systems that provide dedicated interface cards so you can leave your USB port open for MIDI, or printers, or whatever you need.
We expect to see the larger systems continue to lean towards Firewire for the foreseeable future because Firewire has the data bandwidth to easily handle the large amounts of data required to both get multiple channels of audio in and out of the system as well as back in and out to a storage device such as a Firewire hard drive. USB is currently not fast enough to do all that for more than a couple of tracks at a time. There is a new 2.0 version of USB arriving that is much faster, and will significantly close this gap, but there is also a faster version of Firewire on the horizon as well.
Right now a lot of Mac systems are being configured with a Firewire interface and hard drive, but with a USB MIDI interface. This seems to be the best of both worlds and provides a great deal of power and stability.
As with all things computer this is a moving target and you should check with your Sweetwater Sales Engineer about your specific circumstances and goals before you do anything.