i would go with the .357magnum its faster to point and shoot then chambering,shouldering,aiming, and firing.
id recommend sum Corban DPX rounds, a VERY effectince round especially in
a .357mag. the penetration is deep & the terminal ballistics is what you need
to put that mountain lion down ( if need be)
im a dog owner as well so i understand where you mother is coming from
id do the same