fire wire a (400Mbps) is theoretically slightly slower than usb2(480Mbps) . in truth they are practically identical. firewire b (800Mbps) is the new generation firewire : I have not yet personally seen anything that uses it but it is backwards compatible
If it is not loudly written all over your card it is probably ( the more than sufficient) firewire a
Mbps =megabits per second 400Mbps is roughly equal to 50 Megabytes per second much faster than scsci of old and certainly more than adequate for video work as we know it.
http://www.glyphtech.com/site/technology_firewire.html
Seroiusly they are many factors which affect the transfer of data . faster processors, differing archtecture of processor and mother board chipsets, many of which we may have little control over. If it is fragmented even an ultrafast HD in a super duper PC will be slow as a dog
Unlesss you use older kit you are more than likely to get great acceptable results