Vienna
If it works with your time and your budget, try to catch a performance at the Musikverein in Vienna -- preferably in the Grosser Saal. It's gorgeous and is considered one of the top performance halls in the world. The Concentus Musicus Wien performance on Nov 19 at 15:30 catches my eye. Tickets go on sale to the general public on 9/26. The group plays period-appropriate instruments during its performances, so you hear the music the way it was originally performed. I've seen them perform three times, including at the Salzburg Festival in 2014 and twice at the Musikverein since then. Top-notch musicians. Since it's a matinee performance, hopefully the price will be reasonable. If that performance does not work for you or interest you, there are a number of other performances at the Musikverein while you are there. https://www.musikverein.at/en
The Vivaldi concert at Karlskirche should be an excellent performance in a nice environment, too. As an aside, it's cool to hang out at front of Karlskirche in the evenings just to watch the world go by. The group that performs at Karlskirche also uses period instruments.
Salzburg
Salzburg is filled with skilled musicians, too. I have done the fortress concert twice -- once in May and once a few years ago during Christmas market time. I'm sorry you will miss it! I have seen nothing but good reports about the performances at Schloss Mirabell. There are many fans on the forum of the performance at St Peter's, too. My understanding is performances at St Peter's are in period costumes, but the skill of the musicians is very good. The Mozarteum often has quality performances, but I don't see anything scheduled for the time you will be there. I also don't see anything of interest on the Salzburger Kulturvereinigung calendar.