IIRC the Montanas were supposed to have the Mk.II 16” 50cal guns, while the Iowas had the Mk.7 lightweight ones.
The Montanas were also supposed to be more heavily armored than the Iowas, reverting to a 27kt top speed.
The Montana class was designed to go toe-to-toe against the Yamato class. So, speed was sacrificed for guns and armor. But, the battleship had become obsolete. Aircraft carriers and submarines ruled. So, the Navy cancelled the Montana class, and kept producing fast Iowa class battleships, capable of escorting carrier strike fleets, for a while.