I wouldn't even consider the 5.7x28 as a defensive round, not even for a secondary gun. It'd be a fun little plinker to play with, no doubt, but I'm not going to trust my life to something punching that size of a hole.
Personally, I'm still a fan of the 10mm, works for bears, mountain lions and two legged critters. It has great penetration capability with the right bullets and I'm a big believer in penetration, I want an exit wound. There's no point in having a bullet stop half way through its target. If the bullet goes in throught the sternum and exits taking pieces of vertebrae with it, so much the better.
That's my biggest complaint about the 45 ACP, I like a bit more penetration. A 460 Rowland might be about perfect.
