Many people have been clamoring for this topic over at the secret blog.
My views are simple: we have too few cats in the world, relative to dogs. Dogs, for reasons of temperament, can in essence precommit to being our slaves. (As long as they are not Irish Setters.) That makes us more willing to create or support an additional dog. The quantity of dogs is nearly Pareto optimal, although their emotional slavery to us raises ethical questions about the distribution of power in the relationship.
A cat cannot “promise,” genetically or otherwise, that her kittens will become your slaves, if only you don’t neuter her. The kittens never come about, or they meet a cruel fate rather quickly.
If you must support the life of either a cat or a dog, choose the undervalued cat. This argument requires only that the cat gets some value out of being alive, and that value should carry some weight in our all-things-considered moral calculations.
More generally, you should go around helping the (undervalued) people who insult you, or the people who otherwise signal their independence from you. The craven are already being helped quite a bit.
The economics of cats