Depending on the size of the snake, I can hit it with a regular round. I shot a spent shotgun shell from 20 yards or so today with my Ruger 10/22. I can do the same from 10 yards or so with my .45acp.

I say practice with the ammo you regularly carry so you don't have to worry about switching all the time. Also, if you're carrying to protect from bears or other large animals, you might not have time to get more than one shot off. And a bear or other decent-sized animal isn't even going to feel those pellets.