For availability and all around usefulness in the lower 48, I'd probably pic the .357. You can use light .38 Spl wadcutters for practice and training if recoil sensitivity is a factor, and go to the big magnum for SD, HD, or close in deer and hog hunting. It's a pretty all around cartridge.
But if that .45 Colt came with an add'l cylinder chambered in .45 ACP, I'd have to ponder long and hard, especially if it were housed in one of Ruger's new Flat Top Blackhawks, the stainless steel one.
Best Regards, Rod
Our Flag does not fly because the wind blows against it,
It proudly waves because a soldier's dying breath blows upon it.
USAF Forward Air Controller, 5th Spl Forces,
An Loc, lll Corps, RVN, 69-70, Vietnam Vet '69-'73