I have a Linksys E4200 and a DIR-655. The Linksys E4200 is my main router and is in the back of my house, and the DIR-655 is configured as an access point for the front of my house. Both are configured for wireless-g and n, same SSID, different channels (1 and 11). Works very well to cover my whole house with an excellent signal, thus a faster wireless connection pretty much everywhere. The DIR-655 is connected "hard-wired" to the main router via MoCA, works really well for internet access.
I don't have any wireless-b clients, just g and n, but if I did, it would make sense to me to get them off my main router. On the other hand, if you needed better coverage spatially, then you might have the DIR-655 handle b, g, and n clients, assuming that it was always able to work with the wireless-b clients. And the Asus handle just wireless-g and n clients.
You might experiment some and see what works for you.