I utterly hate the Comcast router; There is a spot reserved in hell for that piece of plastic. The Motorola surfboards are the best modems you can buy at a great price and I have never had any issues with mine.
I've always used my own modem with Comcast but be prepared for them to be a little reluctant to set it up (you have to call them and give them the MAC address). Also anytime you call in because of a problem, they will start to question your hardware setup NO MATTER WHAT you tell them. You could be a hardware/software developer at Motorola and explode their brains with your technical knowledge, and they would still doubt your setup.