Tough choices! We have been on 3 Christmas market river cruises, 2 on the Danube. Both were Nuremburg to Budapest (or reverse). Vienna is wonderful. Two days would be great. Many markets in town, we only went to the big one by the town hall, but with 2 days you could go to multiple markets. Have not been to Bratislava or Linz, both would be appealing to us. However, Regensburg has the most wonderful Christmas market. It is called the Romantic Christmas Market and it is at the Thurn and Taxis palace (on the grounds). You do have to pay to get in this one, but I think it was only about 4 euros each. It is absolutely beautiful, fresh evergreen branches cover the booths and everything with white lights. Also, lots of fire pots burning lovely smelling wood. The items at this market seemed a little nicer and a little more unique than some of the other markets. Just a lovely experience. If you are interested in WW2 history, you definitely want to go to Nuremburg and take a WW2 history tour. The rally grounds are there as well as the courthouse. There is a Documentation Center there that teaches about the rise of fascism, absolutely incredible. We went back to Nuremberg for a second time just to spend more time in this museum. Passau is a very pretty town with a decent market, but our second time thru we opted for a day trip to Salzburg. One of our best optional tours anywhere. I would be happy to answer any questions you have if I can. I don't think you can go wrong with either itinerary.