Both are cool, my childhood pet was a cat and he made it to 18, awesome cat. My current pet is a dog and he's great too. Dogs are more useful though, that's for sure. If someone broke into my house, my dog would fuck them up for sure. There's a reason we've kept dogs as companions for so long, not just dogs, wolves too before them. They just have better senses than us and a loyal dog will die fighting at your side and that's pretty cool.
Also all dogs under 20 pounds are considered cats.