It seems there is a fair amount of misconception out there in the coffee drinking world about the differences between dark and light roasted coffees so lets explore the myths and the facts.
First of all, what is the difference between light and dark roast? Of course, a lot depends on the type of bean but in general, a dark roast is roasted longer than a light roast.
Now, what does that mean for our cup of coffee?
Most people associate a dark roasted coffee to a strong cup of coffee and it is assumed that the strong cup of coffee will wake you up and get you going because it is obviously chock full of caffeine. This is myth number one. The longer a coffee bean spends in the roaster, the more natural caffeine burns off and the less is left in your cup. I like to compare dark coffee to dark beer like Guinness; it has a huge flavor and a relatively weak punch. On the other side of that coin, however, is the notion that mentally, a big, dark and flavorful cup of coffee is such a burst that it still tends to wake you up by shear force.
Another misconception is that dark roasted coffee is higher in acid. Though this is a sweeping generalization and depends heavily on the natural acid content of each coffee bean, it is most often the case that darker roasted coffee has less acidity than lighter roasted coffee for the same reason as above; a longer roast burns away more acid. Still, people often notice that dark roasted coffee is hard on their stomach; this is likely due the type of bean or higher levels of carbon, not acidity due to roasting process.
Lighter roasted coffee often highlights more of the natural flavor nuances of the specific coffee bean which come from the oils, acids and fibers inside. Each variety of coffee bean can be best highlighted with different roast levels but in general, the darker the roast-the less nuance and the more "smokiness". Choosing coffee strictly by dark or light roast leaves out a lot of important characteristics that might lead to a more enjoyable cup. Ask your barista or your roaster to help guide you to a coffee that fits all of your coffee needs--for my cup, I like a medium roast, full bodied, medium acidity coffee with fruit forward flavors and a cocoa mouth-feel; this usually ends up being a central or eastern African bean such as the Blanchard's Ethiopian Harrar.