My husband (the car guy) much preferred the Mercedes Museum to the BMW-Welt. I've only done Mercedes and Porsche, and Mercedes is a world-class museum, possibly the best car museum in the world.
Do you have to go to RodT? Would you consider staying in a different quaint city with an intact historical core? Esslingen was, like Rothenburg, a Free Imperial City of the Holy Roman Empire. It was the most important city during the middle ages between Ulm and Strasbourg. Unlike Rothenburg, it's historic core was not seriously damaged during WWII, and unlike Rothenburg it is still a real working city, which means yes, it's not perfectly preserved in amber, you also won't be dealing with throngs of tourist buses, just a real slice of life in Germany.
Another nearby option would be Tubingen, home to Germany's 2nd oldest University that still gives the old city a youthful vibe today. It's very close to Hohenzollern Castle which is well-worth a visit. Tubingen has a castle of it's own. Some people say Rothenburg is a must-see, but with all the beautiful historic cities in Germany, I disagree, and think it only makes sense if it's on the way between two other locations or if you're doing the Romantic Road specifically.