What's your primary hunting weapon? If it's a rifle, I'd say you're better off with it than any handgun.
Kinda hard to bring a rifle to bear on a cat that is on your back, trying to get a good bit on your neck/ base of your skull.... ....... having been the target of a bobcat (case of mistaken identity on the cat's part: I was cammoed up (turkey hunting) and motionless save for my hands): Cats are very stealthy, and you won't likely see them until they are pouncing. I didn't see this one till a split second before he pounced from 15-20 feet out..... and he came from the side, as I had a big tree to my back. I got my shotgun around in time for him to stop about 6" from the muzzle. This was in late march in a SW Nebraska cottonwood filled creek bottom, with little to no undergrowth (smashed down by the now melted winter smows) and a bed of cottonwood leaf litter on the ground, such that you could hear turkeys/deer walking and see 50 to 100 yards
.... that cat sure as hell snuck up on me.
I'm not so sure that the best weapon in the case of a mountain lion on your back would not be a sharp 6" fixed blade knife where you could get to it easily.
I carry my hunting knife AND a handgun when I hunt out there now.