Cons of Cats: They give you condescending looks when you don't do what they want.
They sleep when and where they want, which could be your face, 4am in the morning.
They're better at getting into food, eating goldfish, and getting stuck in areas where you can't get them out.
Cons of Dogs: They have an odd smell(most of them at least).
You have to give them a bath.
You have to walk them, or clean up after them.
Barking is louder than meowing.
They tend to mess on the floor if you don't walk them.

Pros of Cats: Change the kitty litter once in a while, instead of daily.
They clean themselves, and rarely smell.
They don't paw you when you're trying to feed them.
They can actually be quite loving, if you're patient.
They purrrrrrrrrrrrrr!

Pros of Dogs: You can teach them how to do tricks.
Unconditional loyalty.
They're good at cleaning up things you don't want to eat on your plate at dinnertime(only applies to kids).
They will always act like you are god.

Cats are a higher life-form. No other animal is so perfectly adapted to mooch off of mankind.

They get food, shelter, and petting.. and all for catching bice, jumping in laps, playing, sleeping half the time, puring and acting standoffish, things they'd already do in the first place!

PS, Diane Duane, author of several Trek novels, has written two nice books about a group of cat magicians who are the guardians of magical gates, and band together to save the world.