Another way I have looked at it is... The 357 sig is more powerful than a 45 and if you compare similar sized guns ( p220 VS P226) The 226 holds more of a more potent round.
It's more powerful, yes, but it's not more powerful enough to matter.
Defensive handgun calibers are still just defensive handgun calibers. They all have close enough to the same effect on target that you can't really put a value on it, from .380 ACP all the way up to 10mm.
I think you're over-thinking this a bit. Go with a caliber you've already got. Diversity isn't all it's cracked up to be.