Its been 19 years in the Black Hills for us, and that still isn't enough. I don't know how you could do it in two days.
We are close enough to Yellowstone that we have spent a fair amount of time there. A couple of years ago, we spent more time in the Tetons than Yellowstone, and we both loved it.
Honestly, either the Black Hills and surrounding areas (Badlands, Devils Tower, etc.) or Yellowstone/Tetons are worth spending an entire vacation at. It will be tough to see much at either place with the time you have available.
It will be a full days drive from the Black Hills to Yellowstone, and Cody is worth a day itself. Part of the day driving really should include Devils Tower.
