It's certainly not linear, though; I think it will be interesting to see what's better in the long run. It's obviously impossible to measure, due to unpredictable factors such as changes in industry and market and competition.

As a whole, though, I think people, especially in our industry, aren't stupid. Free food would certainly be a more competitive retention factor (than ping pong) -- barring that your company's biggest competitors in terms of hiring aren't also offering free food.