Coffee beans start out green and fairly uninteresting. It’s the roasting that brings out the flavors. The darker the roast, the longer it spends in the roaster.
Light roast advantages include less oil oxidation. The lighter the roast and the fresher the coffee, the lower the oil oxidation. Keeping it in whole bean form also increases the resistance, while grinding it prematurely will oxidize the oil and mar the taste.
Light roasts tend to have more caffeine, as the roasting process degrades caffeine. But caffeine content also depends on the bean; some have more than others.
Both are good, health-wise. Some studies suggest that dark roast has a better effect than light roast on antioxidant capacity in those who drink it. Light roasts tend to be higher in chlorogenic acids, which have been shown to improve subjective mood and ability to focus—even when the coffee is decaf. Medium roasts also have antioxidant effects.
They’re all good. Coffee just works.
