The 6120 has the capability to use 4 bonded downstream channels and 4 upstream. The 6180 has the capability to use 8 bonded downstream channels and 4 upstream. According to Motorola this will allow "data rates of well over 300 Mbps in DOCSIS mode, 400 Mbps in EuroDOCSIS mode for downstream and 120 Mbps upstream." While that all is very impressive, there is not ISP that I know of in the US that will be able to provide a customer with 8 channels down or 4 up. I know Comcast allows bonding of 4 downstream channels and 2 upstream.
I had the SB6120 running for over a year, and liked it. Brighthouse Birmingham converted to DOCSIS 3 several months back, and I was getting 50 M down and 1.9 M up. Well they figuren it out after a couple of months, and without any warning, they locked me down to 15 M with no powerboost I used to have. They don't support powerboost on a DOCSIS modem, so we are screwed. I have spent hours on this. I had to switch bck to an SB5120, and upload was suffering - could not get the 1.9 M i used to have, and am supposed to get. I went and picked up their SB5101, and still reduced upload, but powerboost works, and boosts up to 25M or more. Beware!
A friend of mine just bought by calling the 800 number from this website: http://www.tvcinc.com/ip-networking/customer-premise-equipment/sb6180-surfboard-docsis-30-cable-modem/, I will see how it works for him & order it too if it is as fast as promised. We both pay for Comcast highest speed Internet, if this thing is as fast as advertised Amazon needs to offer it, Motorola website says its for home or office and that download speed makes a huge difference when sharing wifi.