The Economist.com by way of Dan Hare.
Clear advantage here to democracies, although China's gap will shrink dramatically with the generational shift coming up (Hu to Xi).
It's a double-whammy: Democracies get to choose and they choose leaders not too far off from the median, and autocracies are more likely in underdeveloped societies, which - by definition - tend to be youth-skewed (poverty tends to cause babies, just like income growth tends to act as birth control).