How many days (minium) do you need in these two cities to see the main sights?

What is the best way to travel between the two cities?

I'm sure there are lots of opinions, but if I were planning a return trip, I'd probably try for about 4-5 days in each city, with a night train for the trip in between. You could certainly see the high points in less, but I like to have time to enjoy the ambience of a city and to maybe follow up on something I didn't know about before I got there, like a local flea market or a day trip. This often means not pushing the itinerary too hard.