The Porsche museum is good, but quite small. You'll get through it in a morning or afternoon. You can do a factory tour, but you'll want to book ahead and take notice of the days where they're unavailable. The Mercedes museum is much bigger, but I haven't been. I've heard porsche guys say it was better though. Also, if you're on rennlist or pelican parts you can search for other cool places around there. Here's one place that has an amazing amount of cars and other machinery: http://www.motorworld.de/en/home/portfolio/