I agree with the well known quote by I believe Clint Smith that JGGUNS mentioned. But there are a number of things to consider, and no one gun is the best choice for all situations. Also you need to be aware of the environment you will be in when you may have to deploy your firearm.
Is there the possibility of being surprised and the attacker is on top of you while in the building of your business? Will you be transporting money in a vehicle? Will there be customers in your lobby at your business? Thats just a few questions the come to mind right away.
Also, this old adage of racking a shotgun is going to payoff in a big way is a sure way to get killed. Any dedicated criminal will not be deterred by the sound of a shotgun racking, and frankly by them hearing that you may have given up some tactical advantage by letting them know you're armed. With that said, a shotgun is a very good weapon for moderate distances....for close quarter combat it is a poor choice for many (those not well versed in weapon retention) like any long arm, as it lends itself to you becoming disarmed. Also for distances that are outside of 25 yards, you need to be careful if using OObuck. Errant pellets can miss their intended target and hit people down range. You are civilly liable for all those rounds that leave your shotgun. The spread pattern on most 12 guage OObuck rounds is 1" per yard. So your spread at 25 yards is about 25". If you aren't perfect with your shot placement, you could have pellets hitting things you don't want to hit. Also at up close you have to worry about over penetration, particularly if the shot is up close. Same with a rifle, over penetration is a potential problem. You can get frangible rounds for the rifle, I don't like them, but they can be helpful in combating over penetration. If the round over penetrates and hits a bystander, hang tight in court. You likely won't be charged criminally, but a wrongful death suit will be costly for your business, and personal life.
Also, you need to know how to fight (hand to hand), and even use a knife. You don't need to be an expert with either, and certainly don't deploy a knife without some basic training, as a knife fighter may take it away from you and shove it up your arse. Since combat, certainly when it starts as close combat, can become really close if you are caught flat footed. So you may need to fight to put some distance between you an the attacker before you can engage with a firearm. Being able to fight with your fists and knives is helpful. A knife with a double edge blade held in a fist in blade down angle allows you to punch with your fist and stab and slash. A very basic knife technique.
On the training note, get some on close quarter combat with a handgun. There are techniques to keep the gun in your possession instead of being disarmed, and still be accurate. If you are gun savy, or are willing to train a lot, having a semi-auto handgun is a good choice. If you are not willing to train alot, then I suggest a revolver for your handgun. Revolvers are just about stone reliable, and come in effective calibers. You lose a little in the way of capacity from an semi-auto, but if a semi-auto has a malfunction and you can't clear it, it will be worthless. If you go with a semi-auto, you need to practice malfunction clearance drills until you can do them in your sleep. You have 3 basic malfunctions that can occur, fail to return to battery, stove pipe, and double feed. The first two are pretty easy to clear quickly. The double feed is a little more difficult, and requires more practice. Also, the quickest way to clear a double feed (not the only way) is to have a second magazine, so if you carry a semi-auto you should always carry at least one extra mag, if not for the extra rounds, but to speed the malfunction clearance.
Sorry for the long diatribe. Actually it is very short for what really needs to be considered for what you are wanting to do. If it were me I would seek out the advice of someone who is an expert on this topic. Even better if they provide training.
For me in your situation, I would have multiple weapons on me, and available to me.
Also, wear body armor.....
Good luck!
