Wi-Fi Modem vs. modem + router

I always recommend a separate modem and router.  The gateways Cox sells/rents are dumbed down for ease of support.  As has been said here, gateways are for customers who want to plug in a device and have Cox support everything, including their WiFi connection.  If you are not at all technical, the gateway may be a good option.  The pros for separate units are:
- Some more advanced functions available with a router are disabled in a gateway.
- You can update the firmware for a separate router when released by the manufacturer.  With a gateway, Cox updates the firmware after they approve it, which means you may never get firmware updates released by the manufacturer.
- Can be more difficult to isolate problems since with a gateway you can't separate the modem function from the router.
- If you want to upgrade, you can upgrade just the modem or just the router.  No need to replace both.

A good choice is the Arris SB6183 (16x4) or SB6190 (32x8) modem, both Cox approved up to Ultimate paired with a Netgear Night or Asus "ac" router.

I prefer separate, that way if one fails, I only need to replace the one. Plus it allows me more customization for my network, I'm not tied into a particular vendor and/or setup. The corresponding offset to this is that Cox won't be able to support/troubleshoot any wifi issues unless you rent your gateway (combo modem & router) from them.

The other thing is, is that having a separate router allows you to place the router for better reception.  Overall I recommend separate devices, but your mileage may vary.