In an ideal world, neither because they're both such messy animals; scooping a litter box daily that is kept inside your house, versus scooping up the dog crap scattered across your lawn, in the many places where you're liable to step.?
I have three dogs (soon to be four or five, because my show dog's breeder is giving me another puppy or two) and I currently have seven cats in my house (I have nine cats actually, two are at my parents' house for the time being, we get a lot of abandoned cats here, many come pregnant sadly), so I get the best of both worlds; cleaning out litter boxes every single day, and scooping up the dog crap in my backyard... hooray. 😛
I prefer the generally independent and quiet nature of a cat, they are not going to beg to go for a walk or go for a car ride and they aren't going to trample your visitors with excitement, and they certainly aren't going to sit in the window and bark excessively at an empty plastic bag flying around in the wind thinking it's another animal..
While I do prefer the outgoing nature and activity of a dog, as well as being a good walking-companion and for overall companionship and loyalty, but dogs require a lot of work, socialization and training regardless of breed or size; also I do not like how unpredictable a dog's temperament can be now-a-days with all the irresponsible breeding taking place, people breeding dogs for the wrong reasons, resulting in dogs with unstable temperaments and aggressive tendencies, so unless you buy from a responsible, reputable breeder you won't know what you have until it's too late and the dog's temperament proves not so sound.
I never had to housebreak a cat, I've never had to leash train a cat, I've never had to socialize a cat with the entire world just to make sure its temperament will be influenced the right way. Cats are easier.