Amazon has the RT-N66u (Ive owned one before) and Wal-Mart (local pickup has the RT-AC56R for $2 cheaper. Is it worth to get newer hardware? or would the RT-N66U still be preferred? Have not seen much on the AC56R much.
RT-AC55U it's MIPS32 720MHZ with 128MB RAM Qualcomm based and Kernel 3.3.x , RT-AC56U is ARM 800MHZ with 256MB RAM Broadcom based and old Kernel 2.6.36.
I think the N66U has better (among the best?) range, but the AC56U wins in every other way, unless your chosen firmware is not ARM compatible. The price is already ludicrously low on the AC56U... so tempting. Best-value, readily available router for quite a while now, from what I have seen.
I picked the AC56 due to the dual core and ram amount over others in the same price point. It has similar specs as the AC68 but with internal antennas and slower AC speed but I have no need for that much wifi speed anyway. Range seems decent but hard for me to tell, in an apartment building way over saturated in 2.4ghz. That's why I finally had to upgrade just to get the 5ghz. In the back room I get like 30mbps on 5ghz and only 2-4mbps on the 2.4ghz due to noise.