With reference to the performance chart linked to. Remember that these tests were all done more than 5 years ago. In that time the various devices and drivers have all been updated and today FireWire 800 and USB2 devices go head to head in the latency department performing equally well in real life circumstances.

Take the RME UFX, I have used this device extensively and it performs equally well in both Firewire 800 and USB 2. down to 48 samples.