It's the exact same thing as the 6120. Both the 6120 and 6121 are DOCSIS 3.0.
If you have an SB5101 or any other DOCSIS 2.0 modem, you may or may not need to upgrade. DOCSIS 3.0 is only required if you get about a 30 Mbps plan or higher. In fact, most ISP's (Comcast) require you to have a DOCSIS 3.0 modem if you have anything above their 20 Mbps plan.
If you don't know what your speed is, go to speedtest.net and check your speed (wired only, some wireless cards are painfully slow). If you get anywhere near 20 Mbps or above, look into your internet plan and ask your ISP if a DOCSIS 3.0 modem is for you.