A cat can live quite happily without going outside .. they are not the easiest pet to keep within the confines of an open back yard. So the yard itself does you no good. A dog, however, needs to romp and get plenty of exercise, so having a back yard would greatly benefit a dog (of any size). Much easier for potty breaks.
It actually depends more on the size of the animal you are considering. I actually know a lot of people with dogs of a smaller size they keep indoors and walk on a regular basis. It is true that cat's are lower maintenance due to the fact they are more self dependent and don't need daily walks. But both are feasible.