If you have a modem/router or gateway type of device furnished to you by your provider......then it acts as if it were a separate modem and a separate router......but both are in the same physical package.
You only want one device acting as a router on a home network (this has always been a fundamental networking rule), so the correct setting for another device connected to the modem/router or gateway would be Bridge Mode. Apple calls this "Off (Bridge Mode)". What this means is that the routing services of the Apple device are turned "off", to allow the router in your modem/router or gateway to function as the router for the entire network.
When you set up an AirPort device using the AirPort Utility setup "wizard", the wizard will detect that there is another router present and will almost always automatically apply the correct Bridge Mode setting to the Apple device. Most users who set up their Apple AirPorts are not even aware that their AirPorts are operating in Bridge Mode.
In the event that the Apple device is not set up in Bridge Mode for some reason with a modem/router, the indicator light on the Apple device will flash amber. When you check to see why this is occurring, AirPort Utility will indicate that there is a Double NAT condition present on the network and advise you change the setting of the Apple device to Bridge Mode.
In theory, it might be possible to run two devices both acting as routers on a network, but there is no reason to needlessly complicate things by doing this. Several reason for this.....one, you have two devices both trying to provide IP addresses to network devices, which will likely cause IP address conflicts. The other reason is to avoid a Double NAT situation on the network.
Double NAT can cause a number of different unpredictable results to occur on a network, among them.......slowed connections, difficult connections between some devices on the network, lost connections between some devices on the network, slower Internet browsing, even difficulty reaching some web sites. And yet, in some cases things work OK with a Double NAT.
The bottom line would be that you would normally want to avoid a Double NAT error on a home network if at all possible. I can't think a single advantage to having Double NAT on the network, even if the network seems to be functioning OK.
Is there any superiority in using the modem/router to assign IP vs. the AE, or vice versa
Not really, in theory. But, it is a good idea to have the device that connects directly to your Internet provider supply the connection credentials, if possible, only to keep things simple.
Unfortunately, the Motorola router is buggy in bridged mode and Comcast seems to try preventing this, by methods such as disabling the option in their own routers
This is common. After all, the modem/router was designed to be just that.....a modem and router on the same chassis. Things are actually more complicated when you try to make a modem/router behave as a simple modem.
Some providers lock down the settings in the modem/routers that they provide to customers, so although it might appear as if the device could be set up to work as a simple modem, the settings cannot be changed. This keeps the end user from messing things up, so the ISP does not have to spend extra time fixing problems that a user caused by "experimenting" with the settings on their modem/router.
The bottom line.....if a user wants his Apple device to function as the main router on his/her network, they need to use a simple modem, not a modem/router or gateway device.
