Rather than attempting to fret out an answer to which is cleaner, a better question to ponder is whether it's a good idea to be swapping spit with a dog. While most of the bacteria in a dog's mouth is of such nature that it's likely be more perilous to the pooch than to those of other species doesn't mean that all of what's in there can be passed from dogs to humans via Rover's slobber, including rabies. The Centers for Disease Control (CDC) say about dogs that: "To best protect yourself from getting sick, thoroughly wash your hands with running water and soap after contact with dogs, dog saliva, or dog feces (stool)."