Most of the stops are fairly small towns so would think that the tours included by the ship is more than sufficient. This summer we were also on AMA with the first half of the itinerary identical to the first half of PARIS.
Heidelberg vs Mainz - I would say that Heidelberg is more interesting and more to see even though it is about a 2 hour bus ride from Frankfurt / Mainz. Mainz is also charming though.
