Just got back from a week in Salzburg/Vienna. 3 nights in Salzburg, 4 in Vienna. To be honest, we both wish we had 4 nights in Salzburg and 3 in Vienna.

Vienna to us was "fine" but we didn't walk away impressed. It was like Madrid to me. A fine city, but didn't seem to have the charm of Paris or Amsterdam. Plus the Danube canal that runs through the city is covered in Graffiti, which although I can understand is a form of artwork, looked a bit trashy and ruined it for us.

The day trip to Melk was probably the highlight of the trip. We toured the Abbey and the downtown was pretty. The best part was renting a bike from NextBike (it's a bike sharing platform that's 1 Euro/hr) and biking along the Danube. We got to seem some great small villages that we would have never seen.

Salzburg is a much smaller town and can be covered in a day or two, but it is right next to the Bavarian Alps with a number of things to explore including hiking, picturesque lakes, the salt mines, etc.

This is coming from someone who has been to Europe 5x in the last 4 years, so I'm a bit tired of palaces and art and natural history museums. If its been awhile since you've been to Europe, Vienna may have more charm for you. When you're like me and trying to compare it to Paris or Amsterdam, I just found it lacking.