Personally I would suggest going in early December.

I don't think you'll find that a few weeks difference means a large fluctuation in temperature; it might be a little colder on average, but it's not like you're deciding between visiting in fall vs. winter.

In terms of the costs for flights, it probably depends on how early in December you are traveling. I've typically found that holiday fare increases don't usually start until the middle of the month, so traveling in the first two weeks of December shouldn't be significantly different than travel in November.

Hotel prices may be a bit higher due to the Christmas markets, but with advance planning I don't think you should see too much of a change in price between Nov and Dec.