So many choices, and beginning of December is also a very good time to find cheap places to stay and discounts in many places. Amsterdam is beautiful at this time of year, and it is usually quite an expensive place to stay in a regular hotel (the hostels are usually about the same price, but give more discounts in winter), but at this time of year you can almost have the whole city for free. How much are you hoping to spend? It is a good time to see the city without too many tourists, because most of the year Amsterdam is very full of tourists. Every weekend too, but early December and all of January from new year until the beginning of march is very cheap and quieter.
What kind of things do you like to see? The nature, the museums, the parties....?
I just traveled to Rome for the first time last month and it was an amazing experience. If you need warmer winter weather than Amsterdam it is a place to go! I would love to go back, and it looks affordable in the first 2 weeks of the month, many discounts, etc. Both cities are excellent for someone traveling alone, and easy to make friends too!
I have been to Norway in the winter and Denmark too, and of course they are even colder than Holland, but beautiful places, and at this time of year it is the only time when the accommodations are at their cheapest too. Everything else is so expensive there, this must be the most affordable time of year to come and at least save on a place to stay. Norway is full of nature's beauty but also people who like to party.
Depends what you like and the weather you can handle, also your budget. Are you trying to see more than one place or only one country or city?