Newer hardware, newer SDK, better RF design. Overall, better than the sum of the differences.
No 8 Port Asus router is worth considering for the LAN ports. Eventually buggy, or buggy from the start. Not worth taking a chance on one for the $20 cost of a separate 'real' switch.
Here is why I ditched my RT-AX88U for (eventually) 2x RT-AX86Us in wired (2.5GbE) backhaul mode.
RT-AX86U vs. RT-AX88U
386.1 Final 2x RT-AX86Us 2.5GbE Backhaul
Although I haven't done a direct comparison, I'd bet even the RT-AX68U is a better
router too (and the WiFi isn't bad either). Again, newer hardware, newer SDK, newer RF designs/assumptions.
Report - 2x RT-AX68U upgrade over 2x RT-AC86U in wireless backhaul mode
The other side of the coin is that I have installed many RT-AX88Us for customers and they're still going without issues. Still not at RT-AX86U levels though (of course).
I've picked up a couple of GT-AX6000 routers and will be comparing them to the year and a half old RT-AX86Us soon. There may be a new king to be crowned. Stay tuned.