A mountain gun is just that......a mountain gun. It is not a range gun, not a target gun, not a plinker. It is a mountain gun. IMO it is designed to be carried in the mountains or anywhere you may be required to defend yourself from large game that may wish to do you harm or even eat you.
They are short of barrel, thin as possible and light as can be and still perform and remain reliable.....so they can be carried and be with you at all times.
The only factor with regard to caliber is where and what mountains. It you are hunting/hiking/traveling in the southwest deserts, a 357 will probably suffice. If you are in the northwest or northeast for that matter, a 41 or 44 would probably be a better choice. If you are hunting/hiking/traveling north the lower 48, the 44 magnum would be the only choice, as a minimum. The question does not include calibers larger or more powerful than the 44 mag, so the choice is obvious.
In summary, no matter where you travel, why not carry the max mountain gun, other than ammo is somewhat heavier. When I purchased my MG back in the early days, I ONLY considered the 44 for the reasons listed above.
I just today read a newspaper article on a geologist in the field in Alaska just north of Anchorage. The incident occurred this week. He was in a remote area which required him to be flown in by helocopter and extracted in the same manner each day. He was in a clearing which he was cleaning up to allow the pilot, due any time, to land. He was suddenly confronted by a grizzly bear. It attacked him and he fired one shot from the 357 magnum he was carrying before it reached him. He was uncertain if he hit the bear but noticed no change in the bear and immediately went into defense. He dropped to the ground, rolled into a ball, covered his neck with his hands and then was struck by the bear several times. He remained 'dead' and after only short time (15-20 seconds), the bear left. He remained still for a minute or so, then moved to determine how injuried he was. Unfortunately, the bear was not out of sight and when he moved, the bear charged him again from 30/40 feet. He fired two more 357 rounds at the bear, then went into the same rolled up position. The bear mauled him again, this time more agressively and for a longer period time until it again left.
This time he waited for 5/10 minutes but when he attempted to move, he could not. He knew he was seriously injured and bleeding profusely. He could not find his gun, could not stand, and knowing the helo was due to pick him up shortly, luckily found his radio still in his jacket pocket. He summoned the helo, described his attack and the pilot quickly detoured to a camp a few miles away and picked up a person he knew was there that was a paramedic. They together found him and flew him an hour to an Anchorage hospital. His injuries were serious, to his face, head, ears and arms. He will survive, according to the news report. The pilot, a local, said that he did everything right except he moved too soon after the first attack AND carried a poor choice for a weapon. He may have hit the bear every time, but the caliber was not remotely capable of stopping the bear under the best circumstances. The bear was not located at the time of the article so it can not be determined if the victim hit it once, twice, three times.....or at all.
Sorry for the lengthly story, but I believe it makes my point. If you are going to carry a mountain gun for the purpose of self defense, why carry any sub-caliber? He may not have hit it, he may have hit it every time, but regardless, it didn't help him and he is lucky to be alive. Carry the biggest and the best and hope you will never have to use it for it's intended purpose. JMHO