Having a dog and hoping he bites a robber is not a guarantee of thwarting a threat. Two problems here, one, relying on the breed alone is faulty reasoning. My mother-in-law works for a lady who's daughter was approached by a man, he petted her rottweiler, then raped her and the dog had no idea what was going on or what to do. My house was robbed and I have a pit mix and a rottweiler mix. They didn't do ish, and I am fine with that, and here is why: If your dog is not trained to attack on command, and out on command, then if the dog bites someone, the dog is making that decision, not you. So what happens when the dog makes the decision to bite someone he/she shouldn't? You open yourself up to a world of liability when you have a dog that makes his own decisions on who to bite instead of relying on you to make the decision for him/her. Just putting that out there before you go buy a big bad rottie, pit, doberman, or shepherd and hope it bites a thief but not a kid. If you want a dog to attack, make sure he/she is trained to do it and also has a reliable out command. If that is something you are looking for, let me know and I can steer you in the right direction. (I do not attack train dogs yet but I know of a facility that is world renowned in all aspects of training that I could recommend.)
Good luck with this. I hope it doesn't happen again. Being robbed sucks, being robbed at gunpoint I can't even imagine how much worse that is.
-Ryan