Porsche Museum

Is it tricky to get there by train?

No, the S-Bahn stops right at the museum.

From Stuttgart Hauptbahnhof (Main Station) take the S6 in the direction of Weil der Stadt. From there it's four stops to Neuwirtshaus (Porscheplatz). The museum is just a few steps from this stop.

My personal opinion is that the Porsche museum is not really worth it unless you are a serious Porsche fan - lots of variations on the 356 and 911 families. The Mercedes-Benz museum is much more varied and interesting.

My opinion is that the Mercedes Museum is a waste of time unless you are a Mercedes fan as it contains mostly relics of old European taxi cabs and a few over-priced Gullwings.
On the other hand, the Porsche museum is a holy grail experience that contains all variety of gorgeous and classic 356 iterations as well as the follow-on 911 iterations. Porsches racing history is also well represented with a display of many of the most famous cars in modern racing history.
A change of footwear or gumshoes are suggested as the museum floors are often drool covered.
My wife is currently investigating ways to extract me from the museum as I am likely to be overcome with joy and enthusiasm in the presence of so much brilliant automobilia.

Thanks for the information everyone. I'm pretty sure my husband's reaction will be similar to Mack's.