I have try both the sb6120 and the sb6121 and the signal level and speed be the same on both . the sb6121 be a little smaller than the sb6120 but run much warmer than the sb6120 . look like they try to cram too much hardware into too small of a space . my sb6121 do not come with the power save button -lrb- it be list as a optional feature -rrb- , so about the only difference between the two model be the design . both the sb6120 and the sb6121 use the same firmware : label sb612x . so unless you want the different look led light and a smaller and warmer box for a higher price , stay with the sb6120 . as far as speed be concern : the multiple channel bonding on these DOCSIS 3.0 modem be nice for sustained constant speed . it also make it easier for you isp to enforce the speed cap if they choose to do so -lrb- Comcast in my case do -rrb- . I be get 20 + Mbps down and 3 + mbp up on my old Linksys DOCSIS 2.0 modem , however that be the peak when the transfer first start , then it would taper off . the DOCSIS 3.0 modem stay right at the 12\/2 ceiling of my speed tier , but at least the speed stay right up there and do not drop off after a few seconds .