Peronally I would take the entire week at Yellowstone and Grand Tetons. If you have two FULL days, you'll be able to drive the main park roads and stop to see Old Faithful but little else. While much of the park has 35 MPH speed limits, you rarely get to go that speed with all the traffic plus the occasional traffic stoppage for animal sightings. Ofen there is a wait to get a parking spot at the more popular stops. You'll spend a lot of time looking at the vehicle in front of you.
The Black Hills are a long drive and not nearly as scenic IMHO. (Coming from Colorado we understand why they're the Black HILLS not mountains!) ;)