I've now been to three of the manufacturer–run museums in Germany, so here's a breakdown of how you should prioritize your visits if you have no particular affinity for a specific marque or no destination already in mind.
3. Mercedes–Benz Museum, Stuttgart
Fun, but suffers from the unfortunate problem of being in the same city as the Porsche Museum. It's also absolutely gigantic. MB has been around for a while, so they've got quite a bit of history to tell. It took us just shy of two hours to get through, and that was with rushing through the last three sections or so. If you really wanted to get a good look at everything and take full advantage of the extensive audio guide, you could probably spend at least two days here. This is really the museum's biggest shortcoming for a single visit—it's quite easy to get fatigued. If you only have one day, do this after the Porsche Museum, but be prepared for exhaustion.
No one era dominates the museum, which is nice, and they have an excellent series of temporary exhibits. The motor sport section was decently sized, and the prewar section was huge if you're into that sort of thing. There is a tour available of a nearby engine plant, which I did not do. You also receive a complimentary MB Museum lanyard after your visit is over.
The gift/souvenir selection is pretty solid, although most will balk at the absurd prices for the AMG branded gear. Good number of die casts, postcards, prints, etc., most of which avoid being a pointless brand shill and hold some decent enthusiast appeal.
Verdict: Do the Porsche museum first, hit this if you have the time or are planning on more than one day.
2. BMW Museum, Munich
My experience at this one was quite different, since we decided to do a guided tour. The tour took about an hour and a half, but it was nice because my friend and I were the only people who signed up for the tour. As a result of this, our tour was very intimate and the guide could answer any of our specific questions. This museum was also much more focused on a theme (BMW's philosophy) than the others, which presented some interesting information but left the collection somewhat less than comprehensive.
Another advantage of the BMW Museum is their extensive collection of motorcycles and a fair number of aircraft engines—huge appeal for those whose interests are broader than just the cars. All the important models are there, although the M section and motorsport section were surprisingly skimpy in my opinion. There is also the possibility of doing a tour of the nearby assembly plant, but as we found out you usually have to book it quite far in advance. Automobile plants are modern marvels however, so you should make every effort to visit this one.
The museum shop was surprisingly weak. Lots of random items that had nothing to do with BMW, an over–preponderance of books, and an unfortunate selection of borderline tacky branded gear. I usually budget about $60 for cool souvenirs on these occasions, but I left with only a $3 magnet. One bright spot was the selection of vintage owner's manuals (not reproductions) in a multitude of languages—a cool accessory for your classic Beemer.
Verdict: Definitely visit. Expect to learn some neat things about BMW design, check out the new models across the street at BMW Welt, and don't get your hopes up for cool souvenirs.
1. Porsche Museum, Stuttgart
This museum is not only one of the best car museums I've ever been to, but one of the best museums period. To begin with, the architecture is stunning, and everything down to the Porsche Design audio guides has a neat look. The key point is, of course, the collection. It's perfectly sized and impeccably curated, with only the most obscure models escaping display.
The visit feels neither too long nor too short, and caters to everything you might fancy. There are plenty of exhibits about design, more technical engineering exhibits, motor sport exhibits, road car exhibits, and even an exhibit on concept and research cars when I was there. You are not overwhelmed with information, nor are you left wanting more. A well structured experience that leaves no stone unturned without dragging on.
The gift/souvenir selection was also the best by far. All the old standbys are present: books, posters, pictures, a huge die cast selection, and almost any sort of branded gear you can think of (some more subtle than others). Beyond that there are plenty of things that are a little more creative—interesting displays, badges, and other little trinkets like the one I wrote about in one of my earliest Oppo posts.
Verdict: a must see for any enthusiast. If you're planning the German car trip of a lifetime, this is right up there with the Nurburgring, the autobahn, and European delivery. If you're coming to Germany for other reasons, try and add this to your trip. You will be like a kid in a candy store.
If anybody has been to these museums (or one that I haven't), I'd love to hear your own thoughts or stories!