So.. I've done a lot of research on caffeine levels or at least reading various books that skirt the topic. Interestingly, the caffeine amount in a particular roast is slightly different than most peoples conceptions, including my own up until a few months ago.
First off, caffeine levels in dark vs light roasts are often said to be different. From a chemical perspective caffeine is chemically stable to 455 degrees Fahrenheit (it's melting point). Due to this, caffeine is not removed during the roasting process unless you are roasting to a Spanish roast, or burned beyond all recognition. As such, the caffeine inherit in a particular lot of green coffee tends to stay the same amount per bean regardless of roast depth.
Roasting will roughly double bean size, and burn off between 12 and 18% of the total weight of a bean. Due to this, technically, a darker roast has more caffeine by weight than a lighter roast, which is opposite of what I hear a lot of people say.
Now.. on to your question.. If the pods are by weight, I suppose a dark roast will have marginally more caffeine. Where you get a significant difference in caffeine amounts is when you start looking at different types of coffee. Robusta coffees for instance tend to be low grown and have adapted to the environment by having higher concentrations of caffeine. The reason is simply that caffeine is the coffee plants natural pesticide. That being said, Robusta coffees also tend to be very bitter, hence why they are predominantly used in cheap coffee blends in place of arabica. The bitterness is directly related to the elevated caffeine, as well as very high concentrations of CGA.
My recommendation is pick what tastes good, because honestly, you aren't going to get a significantly higher amount of caffeine per roast, or lot of beans.