This is kinda like asking which measurement is most meaningful for the 100 meter sprint - time taken, or time taken relative to weight and height of the runner and their shoe price.Which measurement is meaningful?
It's always been units for NPD and no other measurement matters.
In fact even MS's favoured metric of MAU might not increase much with XB1X depending on what happens with the old console, assuming people are upgrading and the old XB1 goes to users not increasing MAU (old 360 users?).