I think the implication of the OP is;
with the proposed 10 round mag ban there will be guns of the same size, weight and capacity in 9mm and 45.
Without the usual capacity advantages of a 9mm
why wouldn't you upgrade to a 45?
Recoil and cost of ammo are good reasons.
But the capacity argument will disappear.
In full size pistols, 10 for 10 rounds, I'll always choose a .45.