I have the supposed worst dynamo hub, the Sanyo. I can feel the vibration sometimes, but I cannot discern the drag at all. I know it's there, but only because I know it, not because I can detect it. I have one on two bikes. They work great.

I'm lucky that building wheels is easy and fun for me. If you can't do it yet, there are many people here on bikeforums who would love to walk you through it.