Well, dogs want to be around you all the time. And they're always there.... Great if you like attention and have a lot of time, but if you work or go to school full-time, you're doomed. Cats are normally calmer than dogs, and they will still be affectionate, if you're good them. I think I prefer cats, mostly because they don't require as much time or attention. Not to mention that cats are ordinarily WAY smarter. PS Whoever said you can't train a cat, obviously didn't try.
I used to only like dogs, but I got a cat now (from my ex wife) and this little furball is awesome. I only like this cat because she isn't as needy as a dog, but she is still very loyal and friendly. She is right now passed on next to me.