I've read many a post where somebody says that they don't like to take a shot at a deer's neck. Movement, small target, etc. Looks to me like a snake is likely to be much like a deer's neck.
So, closer range, shotgun. And the snakes under consideration in this Everglades deal can be well over ten feet long. I want high odds on a hit at fifteen or twenty yards, and lotsa damage from a hit.