What specs to look for in a router to connect multiple Apple and non-Apple devices efficiently?
It depends what you expect to get out of a router.

I just use an AirPort Extreme, and at any time there are at least 7 devices connected, 3 of which are on the 5ghz band, 3 are on 2.4ghz, and the other 2 are wired.

It's possibly the most reliable router I've had, and is incredibly easy to configure and maintain.

I also have 2 hard drives and a printer connected to it's USB port via a USB hub.

The only issue I have with it is that it doesn't play nicely if I try to set-up an incoming VPN connection on my Windows server, however it does work if I run a VPN on an OS X server I borrowed to test.