Google is your friend, I searched for Motorola SB6120 sb6141 comparison. I'm sure there are more but this was a pretty slide deck.
»www.slideshare.net/pippasymthe/m ··· le-modem
Both modem do DOCSIS 3 which allow the cableco to bond multiple channels. Other then that the only difference is the newer moto unit has a higher theoretical speed. So it all depends on the what speed Internet access you are paying for from your ISP.