The dog lovers win this one:

The intelligence of “a man’s best friend” has evolved at a greater rate than the less social cat over millions of years, scientists at Oxford University have claimed.
It was often thought that the feline pet was smarter than its canine counterpart because it needed less attention but researchers have discovered that cat’s brains are smaller because they are less social.
Later in the article:
The study analysed available data on the brain and body size of over 500 species of living and fossilised mammals. The brains of monkeys grew the most over time followed by horses, dolphins, camels and dogs.
It found that groups of mammals with relatively bigger brains tended to live in stable social groups. The brains of more solitary mammals such as cats, deer and rhino, grew much more slowly during the same period.

This theory is not new. It has been around for sometime and has a name: The Social Brain Hypothesis (it's also called Machiavellian intelligence). The theory is basically this: The larger the group an animal is in, the more mind power it needs. Why? For tracking (check out Theory of Mind). Social animals must keep track of the intentions of others in the group. They have to be other-regarding. The human animal is successful not because of great individuals (George Washington, Henry Ford, Bill Gates) but because of our deep and even biological sociality. The spirit of socialism, as you can see, is so much closer to who we are as humans than the spirit of that other system which currently dominates and distorts all of our affairs.