@ 105 vs 120: I'm not sure the superior range of the 105 is of any use in the real world. It really collides with a low-charged 155. And besides C2 there is the question of targeting. A 120mm mortar has a minimum range of 250 yards or so, a 105mm howitzer a multiple of that. That automatically makes the howitzer a centralized stand-off weapon, whereas a mortar can work with a unit-organic spotter/director.

I do actually see a job for the 105mm caliber, but as a cannon, not as a howitzer. Basically what the Stryker MGS is designed to do - direct heavy fire support.