Basically this, although from what I remember the caffeine difference between light and dark roast is only about 2%.

It is also worth noting that for most people (due to the standard western diet), their daily coffee is one of the few appreciable sources of polyphenols they’re likely to get, and lighter roasts are a better source of that as well.

Aaaand if that weren’t enough, dairy cancels out the health benefits of coffee by reducing bioavailability.