From what I've read, there is not a big advantage except for the model name. Hardware specs for both, similar, and there isn't a huge improvement except for AC. Unless you have devices that will support AC then, get the N66U.
The amount of flash on N66U is 32MB, where 66U has 128mb also.
http://www.smallnetbuilder.com/lanwan/router-charts/bar/111-2_4-ghz-dn-c
I can hit over 50 MB/s with USB 3.0 on the AC68. This is beyond USB 2.0 specs (480 Mbps).
With or without 2.4Ghz enable / option "Reducing USB 3.0 interference" is best to set it enable or disable?
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