A dog and a sign send different messages, and the message of the sign is “the resident is afraid of being robbed”.
EDIT:
I want to make explicit that I am comparing something that gives the impression of a dog without a sign vs a sign with no other evidence of a dog. I am explicitly not commenting on the message sent by a sign combined with other evidence of a dog, just “fake (but, for the sake of argument, convincing) dog” vs. “dog sign”, each alone, as deterrents.