Dog Pros: Companionship, security, playfulness
Dog cons: having to walk it, mess, eats a lot, can be dangerous/vicious if improperly trained
Cat Pros: quiet, doesn't require walking, doesn't eat a whole lot, fluffy and soft.
Cat Cons: Can be bitey/scratchy, quite selfish and don't often want to play, their fur is a bitch to shift when they start malting.
Personally I'd rather have a cat, I like cats. They remind me of me.
I like cats, but I'm really allergic to them. I currently have a dog, but it's an awful, whiny, disgusting thing that absolutely hates me. So I suppose neither. A proper dog wouldn't be so bad though.