Both 9mm and .45acp pretty much stop gaining power once your barrel gets over 12" and they both start losing power after 16". So you want a barrel in the 12-16" range, and for it to not be a SBR that means 16" only.
Personally I went with a Keltec Sub2000 in 9mm. The Sub-2k is a great gun, accurate enough to 100 yards, utterly reliable, and as simple as can be in form and function. I bought the Glock 17 version, so it takes 33 round magazines as well, which are a ton of fun. I also got myself a Glock 17 so I have matching guns for my Bug-Out-Bag. The best part though is that it folds in half so it fits in a backpack, or even on a sling under a long jacket where it deploys in under 5 seconds, including loading.