Iowa's speed would outlast Montana's heavier battery

If both Iowa and Montana were built, which class would the US Navy retain longer?

The Iowa class, because its speed and suitability for carrier battle groups made it more useful in the postwar fleet.

Completed Montana-class battleships would carry more armor and firepower than the Iowas, but postwar service rewarded speed and flexibility. Iowa could keep pace with fast carrier groups and shift quickly between theaters. Those qualities mattered more than the additional heavy-gun power once battleships ceased to form the center of the fleet. The US Navy would therefore be more likely to retain the Iowa class for later service.