Doing it with USB requires extra software and special cables, and may or may not function as a normal network depending on the hardware and software. The basic software doesn't allow just shared drives, you have to use the special software on each PC to do any transfers.
You won't be able to set it up so that the serving PC works as just a plug-in external drive. You will only be able to set it up so that it works as a networked file server, but Firewire can give you a second network connection.
Firewire has built-in capabilities for TCP/IP networking, by default Windows enables the port for networking and you can plug it in and go. Even if neither PC is acting as a DHCP server, the two ports ought to do the automatic IP assignment thing, which should allow them to communicate, and then the drives can be shared just like any network, and you could even run an entire home network that way; Firewire ports all act like bridge devices or hubs automatically. I was able to network my laptop to my main PC using an external drive with two ports. I did manually assign the IPs though, I'm not totally sure if Windows will automatically assign them, as it always shows the port as Connected, as if it's plugged into a hub. At the worst, you just assign every computer with an IP on its Firewire port.
The only issue is the maximum Firewire cable length, limited to 4.5 meters without a hub/repeater.
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