The question of AC vs AX is not limited to transferring large files. You have different frequency options, you have different compatibility, ... Gaming, streaming and other services may factor in. Do you want something for "now" or something that may last longer in the "future". Factor in the price difference to see what works best for you, but hopefully think at least 2 years down the road too.
The newest routers aren't about the fastest/maximum throughput. Everything else from the components used, to the firmware options enabled deep in the boot process has been optimized over what has come before.
If the budget includes both the RT-AC86U and the RT-AX86U, the RT-AC86U is not the solid buying option today going forward.
The improvements made to the RT-AX86U including the SoC, Wi-Fi, and firmware (designs and implementations), in total, make it the best Asus router that is RMerlin supported for over half a year now. So, not only better but for me 'effectively' proven too.