Is there any reason to pick the axe version if I have the ax86u router? I know that this router is not an axe router, but will there be better speeds with the newer card regardless?
Not likely. And you may be disappointed with the AX58BT as well as it may not perform substantially better than what you have now. I have an Intel AC9260 WIFI adapter that performs as well as the Intel AX200 adapter in my laptop.