Most times the dog, unless it is just like a second long death scene. The dog would have to be a minor character the audience knows really nothing about. The character would have to be like one of the lovable main characters that the audience is very emotionally attached to.
But still...I'd mostly be more upset over the dog.
It depends on how well the characters have been developed.
Without character development on the part of the human, I feel that most people immediately gravitate toward the dog. The dog isn't really an individual in most of those cases; he's more like a symbol. "This dog is like my dog." So there's an emotional connection to him without even having to get to know him. A human character (generally) wouldn't elicit this reaction. He would just be some person that the viewer doesn't know, has no emotional connection to, and doesn't care about. With actual character development, though, this may change (or it may not, depending on if the viewer actually *likes* the human character).
Mostly because the dog is usually innocent and the human you could determine that they deserved it or they were mean or something. >.> Not that I am not sad when someone dies, but I am almost always sad when a dog dies.