Neither understands what you see in cats. Dogs miss you when you're gone. Dogs feel guilty when they've done something wrong. Dogs are very direct about wanting to go out. You can train a dog.
Dogs are easy to buy for.
The worst social disease you can get from dogs is fleas. Dogs mean it when they kiss you. A dog loves you when you leave your clothes on the floor. A dog limits its time in the bathroom to a quick drink. A dog never expects you to telephone.