I agree with Xzibit and you should check your local Comcast website for their list of compatible devices.
And FTR, I don't like "residential gateway" devices - integrated devices that include the router and modem in the same box. That's too many devices in one box for me (router, WAP - wireless access point, 4-port Ethernet switch, and modem, and maybe VoIP). I like having a separate wireless router and modem. In this way, should my ISP require a new DOCSIS version, for example, I only have to buy a new modem. Or should my wireless range be inadequate, I only have to get a new wireless router.
I've had great service from Motorola/Arris modems. And it is good to have someone else responsible. But note the excellent
Motorola MB8600 32x8 cable modem can be had from Amazon for $168. In less than 2 years at $15/month, you would pay double that cost in rental fees.
Another problem with ISP provided devices is they typically include back doors into those devices. They are NOT for nefarious deeds. They typically are to allow access for strangers in your neighborhood who are customers of the same ISP to piggy-back onto to your connection. In effect, your ISP turns your wireless modem into a free "hotspot" for their customers visiting your area. To learn more, read about
Xfinity Wifi (other ISPs have similar services). I don't like that idea and I just assume that eventually a bad guy will figure out how to hack that back door, crack the firewall between my network and the public network, and get into my network.
These guests are not sharing your bandwidth. This is above what your service contract allows for. But your ISP and these guests are using your electricity, even if that is just a tiny bit.