Theoretically, Firewire 800 is not faster than gigabit ethernet. Its 1000Mbps vs. 800Mbps. Beyond that I'm not the expert I should be (Physics degree). I should be able to talk about attenuation and the (possibly) different speeds you would get with a direct firewire link, a firewire hub or an ethernet network but I'm afraid that would require alot of revision, then research on my part. I will suggest that based on the fact gigabit ethernet is a much more common medium than firewire 800 in networking, it should be better refined and therefore faster. Remember they are using the same TCP/IP protocols, so only the infrastructure between the cables (or the quality of the cabling which I have no reason to doubt) should really make any difference.
If you need top speed, you need fibre channel. Better take a loan out.