Do you have the time and energy to take the dog for a minimum of a once daily walk? Do enjoy a pet that interacts and plays with you or do you prefer a pet that sits quietly at your side?
Do you like to take your pet with you - if so, a dog may be a better choice. Do you want a pet for protection? If so, a dog would be a better choice.
Do you have the interest, time and other resources to properly train a pet? If not, perhaps a dog is a better choice.
Do you detest barking? If so, perhaps a cat is a better choice.
Are you an outdoors person who likes to take your pet with you on walks and adventures? If so, a dog might be a better choice.
Do you have cat allergies? If so, a dog might be a better choice.
Do you have a dog or cat that doesn't get along with dogs or cats? Then this is a consideration.
Dogs and cats are individuals. Some cats are dog-like and some dogs are cat-like. I think it's best to pick a pet based on what you want from the pet, what kind of relationship you desire, and what your ability is to meet the specific needs of either a canine or a feline. Both cats and dogs make wonderful companions, in my opinion.