Dogs have been specifically trained by humans to be fiercely loyal since the first wolf was domesticated. This has resulted in an intense loyalty as well as love that has travelled down the dogs' (I guess you would call it a family chain? For the life of me I can't remember the term) into all breeds that I have heard of. Other than their unconditional love (unless emotional trauma is involved), dogs still tend to be highly moldable, so you can train them to do what you want within reason, be it gaurd dog, or just something to love. Also, I heard about this study (can't cite it) about how people with dogs tend to be able to vent their sadness to them, which makes them feel better afterwards.
Cats definately have the cuteness factor when they are younger, but that's not the only thing they are good for. Cats can also be used to clear your house (and the surrounding area) of vermin. Also, lets not forget how adorable kittens are when they are playing ^-^. Anyway, their cuteness soon fades as they grow older, and they basically just lay around inside or spend all their time outside, and you'll only see them a couple of times a day. Also I heard (can't cite this either, but I would give it less credibility than the dog study) that cats don't love their owner, just the fact that their owner feeds them and takes care of them.
All in all, even though I have had both species as pets and my favorite pet was even a cat, I like dogs more.