http://www.smallnetbuilder.com/lanwan/router-charts/bar/118-2_4-ghz-profile-up
In the Charts, you can see that the RT-N66U in the 2.4GHz band has the most throughput at the farthest distance (actually; the highest attenuation), indicating it will have the best range.
When you compare the 5GHz band though, the RT-AC68U is almost 40% better throughput.
http://www.smallnetbuilder.com/lanwan/router-charts/bar/116-5-ghz-updn-c
Overall, in an actual setting; I think the RT-AC68U is the best router choice right now. Especially considering there is a mere $60 difference between the N66U and the AC68U (here).
Even keeping in mind that the numbers the Charts are showing us are with an AC client and not an N client. Eventually, we will all move to AC clients within the lifetime of the router at least.
Best option would be to pick up both of them and try them one after the other in your setup. Return the lesser one.
I haven't had a chance to use an RT-AC68U yet; but the RT-AC66U and the RT-AC56U I have used (customer's equipment), they both make my RT-N66U look old tech by comparison (even with my N client laptop). Looking forward to testing one soon.
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