which chipset doesn't matter actually.
what's most important is that under a long term usage, will the router heat up badly, the design of the router ventilation.
How stable and effectively it can run without restarting occasionally is the most important part.
I've read on TP link forums , some users are complaining their AX model still run into restarts, or lagging. Could be firmware or hardware issues, so in the end no solve all method, just trial and run/
But suggestion is to get ethernet backhaul if you want full signal + speed coverage.
Buying a MR90x might be able to cover whole floor, but I doubt you'll get full Wifi signal coverage on the 2nd floor, and even if you get, your speed is going to suffer tremendously.
So best option to get those ethernet conduit and power plug socket built into the walls and placed strategically installed at mid point of the floor to make sure you can place either mesh/router at the right places.
This way you won't have to break your head over which location to put your router even in the future. Because at the right spot with necessary existing cabling, you are future proofed for the next 5 years I dare say.
Make me more confused to buy which one. But what you said is correct. How long it can last and heat concerns.
Need to do more research what is the best.
Thinking mr90x + one cables router as AP for second floor.