possible predators (all kinds-I live a stone's throw from the Mexican border)
I would go with 45 ACP. I'd want the largest round I can get for chupacabra.
Seriously though, you might want to look into whether or not you can buy a 40 x-change kit then just buy a 357 Sig barrel. I have no experience with the p250 but if its like every other Sig that should be all it takes to change and would give you 2 extras calibers at a lower price than buying two of the kits.