AC66U: 600MHz MIPS single core CPU, 3 year warranty
Below are ARM dual core CPU:
AC68U: 800MHz CPU, 3 year warranty
AC68P (AC68U V2): 1GHz CPU, fixed USB 3.0 issue, not available in HK
AC87U: I will not recommend any non-Broadcom radio router
R7000: 1GHz CPU, 2 year warranty, can also run Merlin firmware unofficially (based on Asus firmware)
R7500: I will not recommend any non-Broadcom radio router
A2004NS is actually very good for the price as long as one doesn't need the WiFi to passthrough a wall, and doesn't mind Korean menu for latest firmware. I use it as WiFi repeater with main router being R7000.