I'll start by saying that I haven't been to Prague or Budapest. With that said, we did Salzburg, Hallstatt, & Vienna in September. Salzburg was one of our favorite places, ever--we've done Ireland, London, Paris, Amsterdam, and all the major spots in Italy as well as Tuscany, and would easily rank it in our "top 5." Beautiful scenery, charming old town, great bier halls, cozy restaurants. We loved the people and the atmosphere in the city. Lots of easy day trip options from there as well. We also spent a night in Hallstatt on our way from Salzburg to Vienna, and I think it is the most beautiful place I've seen. It was still a high tourist season when we were there, so be forewarned...but at night when the day trippers leave, it is so charming and quaint. We did a tour of the Salt Mine there (said to be the oldest in the world) which was really fun and informative. If you do Hallstatt, stay at the Hotel Grunerbaum--lakeside and beautiful!