If both the .45acp and the 9mm fail to expand in a given shot, the .45 is better because it's wider to start with. If both expand to their theoretical capacity, the .45 is also better. If the 9mm expands well and the .45 doesn't, then probably the 9mm, or at least it's debatable. So I think you're better off with the .45 BUT then the issue of how many rounds both will hold is the other big factor. If your similarly sized 9mm pistol holds 2 or 3 more rounds than the .45, to me, that MORE than evens the debate? I would have to go with the 9mm based on more rounds, as I think the ballistic advantage of the .45 probably doesn't equal the 9mm advantage in capacity?
I'd feel better off with 8 or 9 rounds of 9mm than 6 rounds of .45 in a sub-compact gun, or 17 rounds of 9mm than 13 rounds in a larger pistol. Just IMO.