Dogs have a better sense of smell for the fact that their noises are designed to pick up the slightest sent. Cats do have an amazing sense of smell, but dogs are more tuned into the current environment around them. Police and hunters have been using dogs for their ability to smell better then any other creature for years.
While dogs have an amazing sense of smell, some cats actually have a better sense of smell. Cats have approximately 200 million sense receptors. Fox terriers have 147 million and Daschunds have 125 million sense receptors. So, some dogs can smell better than cats, and some cats can smell better than dogs!