If you are referring to WiFi coverage, I would say N66U is still ok.
I had an N66U and now an AC68U. I would say N66U trumps AC68U in terms of WiFi coverage and even WiFi speed with n devices.
I do not have any AC adapters, but with my n Laptop I could download locally over WiFi at 25 MBps (N66U). Now, with AC68U and same network and Laptop, I can barely go over 19 MBps no matter what channel or WiFi setting I tried. Same location also. Go figure, more money, better specs, but lower WiFi performace.
I would suggest to stay with your N66U if you do not have gigabit internet.