Damage: The amount of damage the weapon inflicts on the target. The first number signifies its damage output without suppression, the second number is the damage output with attached suppression.
Fire Rate: The rate of fire (rounds per minute) of the weapon, the higher the number, the more "burst" the weapon has.
Mobility: Weight penalty when carrying the weapon. Unlike other FPS, particularly CS: Go, Rainbow Six: Siege's encumbrance system takes account every weapon carried (excluding the gadgets) in assigning weight. This means, the speed of the operator stays the same even if he / she is carrying a pistol instead of his / her primary. The higher the number, the more mobile the operator is when equipping the weapon.
Recoil Pattern: The spray direction of the weapon, naturally, shotguns do not have recoil patterns. Learn the recoil patterns so that you can compensate when you are firing the weapons and steer the bullets to your target. This will allow you to immediately kill the target without missing a lot of fire.
Well, they are basically the same and its unwise to use them as the main weapon for kills. For both Montagne and Blitz (and Fuze if you opt for the Ballistic Shield, which is never recommended), choosing between the two handguns is pure preference. I am sure there are advantages and disadvantages of choosing one over the other, their damage output and their recoil patterns are widely different (and in the case of the SMG-11, the entire gun mechanics). Still, all of these are minute differences in the grander scheme of things.
It is rather unnecessary to create guides when it comes to shotguns. In fact, I would choose to create a handgun guide first before even thinking of creating a shotgun guide. Shotguns have only one rule: shoot at very close range. The are no recoil patterns, no difference in damage (technically), and of course, they kick *** whatever kind of shotguns you are using, they work only in close range.