Both areas are a lot of fun in winter!
Fasching is a lot of fun! The festivities surrounding it start about a week before Fat Tuesday, and involve a lot of drinking and carousing. In Munich, there are huge parties at Marienplatz and the various beer halls in town. If you happen to be at a ski area (either in the Bavarian or Austrian Alps), you'll see groups of costumed revelers, skiing from one on-mountain hutte to the next, drinking and partying (beware the end-of-day runs down the mountain, as drinking and skiing are not fast bedfellows).
Since you'll be in the Salzburg region, you're not far from the Salzkammergut and the alpine regions south of the city. The Pongau river valley is particularly beautiful: Hohenwerfen (seen in "The Sound of Music") is really striking, and along with the neighboring towns of Bischofshofen, St. Johann im Pongau, Wagrain and Flachau, makes for a beautiful day of ogling at mountains and enjoying true Alpine scenery. All of these towns are accessible via mass transit (train and bus), and the ski lifts that connect the areas can take you up to some wonderful viewpoints.
You should also consider side trips to Garmisch-Partenkirchen and the high mountains of southern Bavaria: great views and fun little villages. Also worth looking into on the way from Munich to Salzburg is the Chiemsee area, with its castle-on-an-island, as well as the lovely town of Prien.
There will very likely be snow, as this has been a great snow year in the Alps. You're more likely to encounter the white stuff in Salzburg, given its closer proximity to the Alps. Just be prepared for real winter weather, with plenty of layers that will give you versatility (lots of light layers beat just a few bulky ones any day, especially if you're going to walk a lot), and you should be fine.
