I wouldn't recommend using the modem part of the Asus. There is a 95-page thread on the Overclockers forum about Asus problems. It seems to work OK for a few people, but there are many for whom it doesn't.
I have an Asus DSL-N66U (a relative of the AC68 but without AC wireless). Even with last month's firmware upgrade my download speed is around 30Mbps. When I use my OR ECI modem my speed goes back to its usual 60Mbps (BT Infinity 2).
It appears that the Netgear D7000 is a much better bet for a single-box solution, but I haven't tried one on my line.