Its a matter of price to performance between these two. AC86U is kind of the "price king" for all sorts of router features including VPN speed with merlin firmware.
The AX88U has faster internal Broadcom SoCs for the radios that apparently run at 1.5 ghz according to arris marketing but I'm not sure if this has a significant difference over the last generation 800mhz variants in the AC wave 2 chips. I would assume this helps with speed and responsive of the WIFI radio all around.
The main CPU is quite similar on both. Its a dual core vs quad core variant of a 1.8ghz 64 bit A53 ARM design.
Ram is also doubled on the AX88U, but I'm not sure if that will make a difference. I just noticed my specific GT-AC2900 (AC86U Reband) is pushing 400mb use with everything I have enabled..so that should help with newer SW down the road if stuff starts really using RAM.
AX86U might be a better option but its yet to be priced officially in western markets. Should be anywhere from $280-300 based on features and i'd be shocked if ASUS prices it closer to the 88u for the sake of a 2.5g port advantage (88u has more ethernet to compensate).
CPU, Ram, 5G radio are identical to 88U. Only real difference is a newer 3x3 AX design vs the 88U's 4x4 radio for 2.4ghz.
Looks like a $170-200 vs $280-300 price difference at that point which isn't that bad considering the upgrades I guess.
Main Cpu: 2c>4c
Ram: 512mb>1024mb
Radio SoC speed (4x4 5g): 800mhz>1.5ghz
1gb >2.5gb port for file transfer/WAN, should have higher VPN performance.
Pretty much doubles capability on everything, but it comes down to if you "need it" really. I personally do not like the idea of spending over $200 on a router thats typically dated in 4-5 years time.