I can't compare the BMW Museum to the Dachau site - it's not like comparing apples and oranges, more like comparing apples and zebras. I think you'll have to decide what your biggest interest is...
Still, some logistics info might help. You can reach the Dachau site from Munich central station in about 40 minutes. You take the S2 S-Bahn (local train) to Dachau station, and then Bus 726 from the plaza in front of the station. You can take the U-Bahn (subway train) to the BMW museum in about 10 minutes from the central station. Either way, you'll have no problem returning by 19:30.
Good luck!