Since I'm looking for a BUG, would caliber really matter? Grant it if I'm reaching for my BUG then probably the assailant would be close to me. I would think a 40 or a 45 would stop the assailant more quickly than a 9 would. Or perhaps it wouldn't matter. I'm just thinking outloud.
It wouldn't matter. The only way to stop a threat dead in it's tracks guaranteed is a head shot. And a heart shot from any caliber they will still be alive and functional until their brain loses oxygen. They could keel over from pain or shock though and stop.
A BUG is always a good idea, just have to practice drawing with it, should also place your firearms in a position that allows you to draw from either hand.
Just when considering a backup gun, if you wouldn't consider it for a primary then it's probably not a good choice for a backup.