Wireless card always if you can, USB often have problems, especially if your computers working hard and you use WPA or WPA2 security (for heavens sake don't use WEK).
Any card will usually behave itself but you may get better performance if its the same make as the router.
What standard/speed is your router, if its a giz-it its probably not wireless-N which would give the best performance.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates