Well USB maxes at 12mbps, which is shared. So 10baseT would suck nearly all of USB so if you have any other devices, you probably won't even get your full 10bT, plus USB uses cycles. A USB NIC also has none of the features of a PCI NIC Card...plus toss in an extra $5 and you can get 100mbps PCI NICs...then if you ever use it for a LAN you have decent speed. Believe me...transferring a CD image to another computer on your network sucks if you only have 10baseT...
As I'm sure you've seen by the landslide vote it's not a tough call.