I am testing a DOCCSIS D-Link now and so far it has been great, but I'm not finished yet. Many cable companies specifically use DOCSIS certified modems, which is an acronym for "Data Over Cable Service Interface Specification". DOCSIS provides a set of standards by which cable companies can achieve cross-platform functionality in Internet delivery. Most cable companies use this standard as their guide when designing a data ready system, however, I suggest that you contact the provider in your area and see. Cox, Comcast, TW use DOCSIS in their larger markets but I can't say for sure about the smaller systems.
My choice is the Motorola 3100B. The difference isn't noticeable to the end user so much, but the Motorola will tolerate more upstream noise than the Toshiba, RCA, and Samsung. My second choice is the Toshiba. It's solid and performs very well
I've heard good things about the COM21 and 3Com's but haven't got my hands on one yet.
Concerning USB or Ethernet: Since the max x-fer of the cable modem will never reach the throughput of USB it shouldn't matter which one you go with. However, my experience has always been more favorable overall with an Ethernet connection. Maybe it's just me, but I can have either and that is what I use.