Yes, I get a modem as the Comcast wireless modem has wireless hot spots built in and I do not want that.
http://www.comcast.com/wifi/faqs.html

Well, in the Portland, Oregon area, that SB6183 would not be fast enough in some places.
So the 6121 would do just nicely for most of us, some will need to match their download speed to a faster modem that will handle it.