The hardware that interacts directly with your ISP is the modem and, if you use VoIP, an MTA. The router is part of your personal LAN, performs identically regardless of ISP used, and outside of the Comcast realm. Click "hey mods" in the lower right of your post and ask them to move this thread to the Wireless Networking Forum.
To your question:
The WNDR3300 is a fine, simultaneous bg + n router and you will get exactly zero benefit from changing to another 802.11n router. Stay with it until you upgrade to 802.11ac. Most household LAN environments & usages remain some distance away from ac being a worthy move.
If you are suffering poor reception in some parts of your space then post descriptions of problem areas, devices, relative location to router, and size/floors/wall materials of the building. It's an RF signal mitigation issue, not the router, per se.