I've used both USB 2.0 and 400 for capture and both have worked fine for DV. I've used USB for HDV (Sony) and that has worked as well on FCP. It really depends on what you are capturing (DV, DVCAM, DVCpro, HDV), and with what compression (if any). With HD MPEG2 compression, it might be a good idea to go with 400 or 800, because if you drop a "frame" it could be an "I frame" which means you lose a GOP (many frames) not just one frame (even though dropping one frame is unacceptable in the professional video world😱). I would go with firewire just as a safe investment in more advanced technology.
Stick with USB if you are on a tight budget and are editing DV.
Buy firewire for reliability and efficient workflow, and if you want to edit HD.
-Marty