Significant.

USB has a ceiling on speed. There's a certain point at which it simply can't go any faster. With the ethernet link, that ceiling gets raised significantly. - everything has a ceiling, it's just a matter of how fast.

I recently did some testing on my cable modem too and was surprised by the results. Turns out my cable modem at home is faster than my T1 at work! And this was with a Linksys router stitting between the modem and the PC.

Honestly, there isn't a reason to stick with the USB connection to the modem. Ethernet is the fastest, most reliable connection there is. (in this case)

-tg