I have an RT-n66u and have seen it recommended over the ac66u by others as well. The reasoning generally was the the n66u being a bit older product had more mature firmware available for it. If you are using the RT-n66u there is an enhanced version of the AsusWRT firmware available from a fellow that goes by the handle of RMerlin over at smallnetbuilder.com. He even has his own subforum under the Asus wireless forum there. If you like the AsusWRT firmware but would like to see a few extra features in it take a look at the AsusWRT-merlin build. It sits between AsusWRT and Shibby's TomatoUSB Firmwares for the RT-N66U. I have been running it for about a week and so far seems pretty solid.