"I would go with PCI if at all possible. USB is too iffy IMHO, and I wouldn't use it if I had a free PCI slot available. Also, if the two computers are older and only have USB 1, and you get 54g, you'll be limiting your network bandwidth."
That's why it's important to buy good components. I am using my USB Netgear MA101 adapter on this computer because my D-Link DWL-520 PCI adapter is too unstable with it's signal strength. Plus the USB gives me the option to move the receiver if I need to, for better reception. Though I didn't consider the USB 1 point that Redfallon mentioned. If it is USB 1 then you may be better with PCI regardless.
Moral of the story, don't necessarily go with what is on sale this week
