Don't buy either one, adopt one from a shelter. If you want something around to pet occasionally and make it go away when you don't, get a cat. If you want something that will bond with you openly and show great enthusiasm for you, get a dog. Simple!
Cats you can leave and go on vacation for a few days dogs you have to bring to a kennel or something their like children always make sure their safe, bring them potty all hours of the day/night, cats don't have this problem