The USB solutions are restricted to USB's 15mbs speeds, so can get away with 320x240 pixel resolution at something approaching 30 frames per second (depending on how good their usually basic hardware compression is), wheras the PCI card can chuck a *lot* more data through and are usually way better (especially for the various video-in cards).
The USB gadgets are more convenient though, and can be used on the road with an iBook or Powerbook.