All other things being equal, a light roast is going to have more caffeine in it than a dark roast.
That's because the process of roasting beans causes some of the caffeine to escape.
The difference isn't super large, though. One study comparing unroasted and severely roasted beans showed only a 5.4% difference in caffeine content.
The grind, the method of brewing, and the environment where the beans were grown all affect caffeine levels too.
In terms of things we have control over, the real issue is taste.
Unroasted coffee beans taste really nasty.
Roasting the beans allows them to undergo a complex set of reactions that produce brown colors and a multitude of flavor-bearing chemicals.
Bottom line: Don't worry too much about the caffeine content of the roast you choose. The difference in caffeine will be negligible compared to the difference you experience in taste.
Science Writer, Fossil Beastmaster
Contact Alex Kasprak at firstname.lastname@example.org.
Got a confidential tip? Submit it here.