I actually have both here. An RT-AC66U that acts as my main router, and an RT-N66U which used to be my main router, and is now dedicated to FW development.
To be honest, I saw no notable difference in performance between the two routers in my particular setup (note that this will vary for everyone based on their particular environment). Sure, maybe if I were to run benchmarks I might a see a difference of a few mbits here and there. Part of this would be attributed to the fact that the newer router doesn't have a driver that is as mature yet as the older router. But for day-to-day usage, I'd say the two should give the same performance for me. So unless constantly transferring multi-gig amounts of data where saving a few seconds off a transfer matters, it shouldn't be an issue.
The one issue that came up in some reviews (especially the CNet one) was that the RT-AC66U seemed to do poorly in a situation where there was a lot of interference from other routers. That's why the SNB review had better results, as it had a clearer environment. Asus took note of that difference, and are actively working on improving the wireless driver. The first step was the new Enhanced Interference Management option that was recently added. There should also be a wireless driver update coming in the near future which should hopefully help even more improve general stability in such environment.
So bottom line, it mostly comes down to price. Is it worth paying the premium for RT-AC66U? Some people think it isn't. Some are also worried that the fact the 802.11ac standard is still draft could potentially lead to compatibility issues a few years from now (personally, I doubt it).