I would choose the one with better WiFi as a first step.
The performance of those chipsets is comparable, as it seems (won't be great):
In the next step for a x86 based router, a used Futro S920/S940 and a 2-port NIC could fit the budget.
Once you got the x86 router, you can switch the Xiaomi router to a "dumb AP" configuration, which is lighter on the resources.
EDIT: Also worth considering, the AX3000T is only supported in snapshots at the moment.
So if you need a WiFi router, which "just works" it's better to get one supported in the stable release.