From what anicdotal evidence I've read about from it's use in the pacific HE would make more sense based on the targets they were going after.
During the B-25H's brief stint with the 345th they were attacking a lot of ground installations, in fact they didn't like the 75mm cannon much at all as it was not as well suited in their opinion than the massed fire from the .50 cal installation. It also required a higher, longer approach and firing run which made it an easier target for gunners on the ground. On one occasion they were called upon to engage some japanese tanks that were threatening allied positions but by the time they got there they tanks were already knocked out. Dunno if in this case they would have loaded AP or not, my understanding is that the japanese tanks with a few exceptions were relatively lightly armored.
As for shipping targets, for the most part the B-25's were hunting merchant shipping and barges, though they did go after the escorts on occasion or lone destroyers as well. I don't know what a 75mm AP shell would do to these types of targets at any rate. The bombs they were packing did a lot more damage and typically did it down near the waterline. The .50's and HE I would imagine would wreck more havoc up on the upper decks which is really what they were trying to do in order to surpress the gun crews so they could get in close enough to deliver their bombs.
