Which one is better for 4 days in December, Garmish P or Fussen? Better I mean for trains (as we will not be driving), day trips, scenery, restaurants for dinner.
Garmisch, IME, is the winner based on your criteria. You can take a scenic train route called the "Ausserfernbahn" through Tyrolean Austria and back into Germany (Kempten) or a day trips to Mittenwald, Innsbruck, even north to Augsburg (about 1:40.) The Bayern ticket or Werdenfels ticket ( both are daypasses) is good on all local day trips except Innsbruck. Garmisch also issues a guest card that offers free public transit around town and other freebies:
Agreed. The is a little more to do in Garmish-Partenkirchen, plus access to other places is better. Füssen only has one train line, NE to Buchloe. From Garmisch-Partenkirchen you can go in three directions.
There is no train line directly between Füssen and GaP, but there is a two hour bus via Oberammergau, or you can take the Außerfernbahn through the Tirol to Reutte or Pfronten and a bus from there to Füssen. Another scenic bus ride from GaP takes you along the shores of the Walchensee, then over the mountains to Kochelsee. From Kochel, you could take the train to Tutzing, on Starnbergersee, then another one back down to GaP.
GaP is kind of the transit hub for the region. It's the largest Alpine resort in Germany, so there's plenty to do. As noted previously, Füssen is sort of a dead-end.
I suppose you mean December of 2015? The Christmas holiday is the busiest time of the year for GaP. If you've waited this long for 2014, you may struggle to find lodging that fits your criteria.
Do you think 5 nights in GaP will be good? Here is my Plan:
Day 1: Arrive in GaP
Day 2: Go around GaP
Day 3: Visit Neuschwanstein Castle and Linderhof Palace with a quick trip to Oberammergau - I think I will take a tour
Day 4: Day trip to Mittenwald
Day 5: Visit the Zugspitze area.
Day 6: off to Nuremburg
You would probably do better to keep a flexible schedule based on the weather, rather than trying to pigeon-hole your activities this far in advanced. The Zugspitze, in particular, can become a near white-out when a snow storm rolls in (I could barely see 10 meters ahead of me once when skiing up there once).