RE: 9mm or .45
The case for the 9mm:
1. The 9mm is more accurate, so you more likely to hit your target;
2. The 9mm has less recoil, so your follow-up shots are faster; and
3. The 9mm will carry more rounds, so you can fire more rounds.
Those are three large tactical advantages over the .45. If you use hollow-point bullets there is plenty of stopping power and energy transfer to the target, so it is not wimpy.
-MG