But I can get you pointed in the right direction, I hope, by listing a couple relevant factors to consider:
Which animal, dog or human, has a greater capacity for conscious thought, reflection, appreciation, joy, and so on? (Isn't the reason we don't care about killing ants that the ants have so little capacity for thought, and therefore such a limited quality of life? This is why a hamster life is worth more than an ant life. And a dog life is worth more than a hamster life. And a human life is worth more than a dog life.)
Which animal, dog or human, has more invested in its life in terms of personal relationships, family obligations, and so on? Whose death will cause more suffering to his or her loved ones? (Would you rather lose your dog or your brother? The drowning stranger probably has a brother or sister, not to mention a mother and possibly a wife and children.)I'm sure you can come up with some other relevant factors on your own.