You can't depend on expansion, and .45 bullets are essentially pre-expanded, so go for penetration. Fears of "overpentration" notwithstanding, I'll take all the penetration I can get.
Expanding bullets are designed to work within a specific velocity range, so the idea that you will help ensure expansion in short-barreled guns by using a lighter, faster bullet ignores that the ligher bullet is also going slower than it would in a longer barrel, and so also outside the ideal velocity range. All the more reason to go with a heavy bullet, so you at least get good penetration, expansion or no.