If your system has an Ethernet port that is supported by your operating system, you will be fine. If your system has WiFi that is supported by your operating system, you can untether your system by adding a wireless broadband router. I don't bother with USB and special drivers, but it's nice to know I have that option if I ever need it.
This info from Motorola shows the SB5101 without USB and the SB5101U with USB. If you saw a non-U SB5101 with USB, it's possible that it was mislabeled or Motorola changed the specs somewhere along the way.
I was asking myself the same question. I found this on the Motorola site, after a bit of digging: "Capable of downloading up to 2000 times faster than 56k analog phone modems; up to 4 times faster than DOCSIS 2.0 broadband cable modemsOptional Installation Assistant available (models SB5101U/UE) on CD-ROM guides you through installation on the PC " -- SB5101. I would infer from this that the difference is in the CD (the U indicates it has the install help?) and, further, that the 5101 is indeed DOCSIS 3.0 capable, if and when that is rolled out and we want to pay extra for it.