Just my opinion I would not get a gateway. I would keep things seperate because it is easier to trouble shoot when you have problems. And from the sbg 6580 and zoom 5350 they have weak signal strength.
i would buy neither of those gateway, or any gateway at all. Many have issues with compromised performance, missing features, etc. They sould like a good idea, but end up being a problem. Another issue is the firmware is completely under Comcast's control, and you have no control over it. With a seperate cable modem and router, at least you control the router's firmware and can change it as required. As pointed out before, they make trouble shooting difficult because you can't eliminate one side or the other easily on most models.
My advice: get a Motorola SB6120 or SB6121 or Zoon 5341J cable modem and good wireless router that meets your needs.
I have the Motorola SBG 6580 works great bought it last year been running fine . Just turn off in the firewall setting's IP Flood Detection its on by default which is the main reasons for people having problems with it i knew that when i bought it so i have had no problems i like it is awesome with Blast! .
Just got the ZOOM 5350 and haven't had any issues that I didn't have with my previous SB6120 / DIR-655 setup, except dealing with Comcast support. By reviewing this thread, I suspect they don't want to push Router/Modem combos because that means less devices they can rent out. This is like asking a car salesman which car to buy.