What are the differences between the Motorola SB5100, SB5101, and SB5120?

Similar in appearance and features, the SB5101 & SB5120 are not repackaged versions of the SB5100.

The SB5100 uses the Broadcom chipset/processor BCM3348.
The SB5101 uses the Broadcom chipset/processor BCM3349.
The SB5120 uses the Texas Instrument chipset/processor TI4401.

Each modem has its own firmware versions unique to its model.  All other product specifications are the same for all 3 models.

The SB5100 has been discontinued by Motorola, but support is available.
The SB5101 is a current model.
The SB5120 is being phased out.