Use Apple Airport Extreme or Xfinity gateway for wifi?

given that my Airport Extreme is now several years old, does it make more sense to use the newer gateway both as a modem and a router and just use the Airport Extreme for storage of backups, or should I continue using the gateway in bridge mode as a modem only?

It all depends on whether the new gateway will outperform the AirPort Extreme (which sounds as if it is really a Time Capsule). To know whether that might be the case, we would need to take a look at the specifications for the Xfinity gateway. Might you have a link for more information about that topic?

Another possible issue is that Apple expects you to use the AirPort WiFi for backups, so they do not officially support backups over a third party WiFi network.

That being said, backups will likely work this way anyway, but with the downside that Apple will not offer help or support if you have problems backing up over the Xfinity WiFi.

Finally, some gateways can be configured to act as simple modems, and some cannot. You may need to ask Xfinity support whether this is possible with your gateway, and if it is, whether they will support this type of setup.

If you want to use the gateway as a simple modem, it might be easier to simply swap the gateway for an approved modem.

The Cisco gateway is clearly the better performer in terms of potential WiFi speeds and features, but unless your Internet connection plan is faster than 300 Mbps or so, I doubt that you will realize any real benefit speed wise from the new gateway.

The Time Capsule design was introduced almost 5 years ago, and Apple has not made any hardware changes on the TC since that time. So, probably fair to say that the AirPort is getting a bit long in the tooth, yet is still a good basic router. I've used one for the past 4 years or so with no issues at all.

The Cisco may or may not provide better WiFi range than the Time Capsule in your home, so if the TC has limited range, or some perceived "weak spots", the Cisco gateway may do better in that regard. That is something that you will have to test on your own.

As far as whether you could use the Cisco WiFi to back up to the Time Capsule, this will probably work OK, but again, the only way to really know how well.....or if.....this will work is to try it out. If things work fine, you may not really be concerned that Apple frowns on this type of setup.

Good luck!

Please let us know about your findings when and if you can, to help other users who may be looking at the same or similar situation.