With regards to Ethernet Vs Firewire, ethernet is probably easier, if only because there will be more people to help you when something goes wrong.
As far as Quake 3 performance, there shouldn't be any noticeable difference. Quake doesn't push a lot of data, it's more dependent on latency. I've never used IP over Firewire, but I imagine the difference in latency between it and ethernet are negligible for something like quake.