I live with a Golden retriever and three cats. The Retriever is convinced that everyone who comes to the house is there to play with her - if someone broke in, I'm sure she'd "toy" the guy to death. I think you're better off installing an alarm system. On the other hand, my girlfriend's cat kills rabbits. We're open to the possibility that she may also be taking out deer. When she (the cat, not my girlfriend) gets mad and glares at you, it's like a flashback to Pet Sematary.
I choose Dogs they more useful and they will protect you when your in trouble. I have 3 dogs 2 Labrador Retriever and 1 chow chow. My Chow is the boss
Goldfish are really lousy watch dogs. They don't even bark. All they can say is "bubble"... how scary is that to a burglar?