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 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 router
A key weakness of the RT-AC68U has been its inability to make full use of its USB 3.0 port. The most I've gotten is 24 MB/s, despite repeated attempts with different firmware and both enabling and disabling its Reducing USB 3.0 interference setting. So I was happy to find that the 68P seems to have solved the problem.
The hardware fix is in AC68P. It also upgraded the CPU to 1GHz.