Immortals Of Aveum or cod mw3 series x

It really boils down to personal preference doesn't it?
If they provide a physical copy of the game and you don't want it then Call of Duty would have the higher trade in value currently.
It's worth mentioning that the Call of Duty franchise is very mature (it's over 20 years old now!) and will have fully optimised gameplay and features for the Series X.
Immortals Of Aveum is still very much a work in progress (they just released a new update that changes gameplay)