Over all, I'd say a dog is better. Their lapping is more instinctive and comes naturally to them in addition to the larger, rougher, stronger tongue. That said a man (or woman) who knows what they're doing is awfully fucking satisfying. A human tongue can be more articulate. A human is like an artist wielding a small brush delicately and deliberately to paint a picture where as a dog is like a house painter using a wide brush to get maximum heavy coverage. They both do a good job, but it's a different job. I wouldn't turn down either one.