Maybe the additional drag of a sidewall dynamo wouldn't be very significant, given how much energy it takes to propel it. You won't believe how much you'll love the convenience of a dynamo once you try it. To me, the most important thing is the headlight, since headlights use so much more power than taillights. Never worrying about the charge of a headlight's battery is a wonderful thing. And you can actually power two headlights with a dynamo.