Private day tour to Salzburg and/or Fussen?

I'm not big on tours that are easily accomplished independently. If the Ludwig castles are your main aim in Fussen, then a private tour is completely superfluous. Ditto with Salzburg from Munich. It is so easily done with just a map and guidebook. EXCEPT for a SOM tour. Because these include time outside the city, I would recommend a prebooked tour. We really enjoyed Bob's Tours.

Much like CJean I see no reason for somebody to accompany me somewhere that I can really simply do myself. Füssen is nice enough, but only a few streets. Most people go there to see what Ludwig built (and often leave disappointed), but if you want Neuschwanstein you have to use their guide inside. Your guide just waits for you. A waste of money in my opinion.

Salzburg also is very easy to explore on your own, old town, mountain, river, splashy house. But there is a chance that a good one could show you parts of the city you might not find if you didn't want to study, like the cemetery, the bakery, the giant game of chess, history of different buildings and Mozart buildings. And what Americans look for most - Sound of Music (even if it was filmed in Hollywood mostly). A guide, and a bus full of singing tourists would help there.

The train trip from Munich is very easy and I don't see how a guide could help with that part.