USB 1.1 is 12Mbps, Ethernet is 10/100 or 1000Mbps with 10/100 being the most common (1000Mbps equipment is expensive) and PCI is 1048Mbps.  Cable internet services are normally 4Mbps and below.  So having a cable modem on any interface be it USB,  PCI or Ethernet will make no difference in download speed.
I would stay away from a USB modem if at all possible  They are troublesome and flakey, at least in my experience anyways.
And I can't say I've ever heard of a PCI cable modem.  I've only seen Ethernet and USB.