Dogs. I'm very allergic to cats. But its not that. I've had dogs my whole life (if you consider 14 years a life). Plus, i used to have a cat when i was little (until i found out i was allergic and had to go to the ER), but they scratched me and hissed at me a couple times when i sat next to them. My Uncle's cat also scratched me a couple times for no apparent reason... I guess cats think i'm a scratching post or something. >_>
Anyway, i know think dogs are more loyal. Dogs know you as a life partner and as a friend they rely on, while cats are just like "Yeah, so i'm just gonna live in this human's house, and sleep all day. kthxbai." And dogs are usually more active and playful (well when they are young, anyway). I'm not saying cats aren't active. Believe me, my cats were very hyper. It's just that dogs in general are more playful and active.
Well that's just my opinion. I have nothing against cats or cat owners, i just think dogs are slightly better.