Down has the highest insulation ratio to weight of any insulation material. It is washable, natural, renewable and sustainable. So if you need a warm coat, go with down, its worth the investment.
Key things to look for when buying a down jacket is amount of fill. The higher the fill number, higher the ratio of large fluffy down feathers and a very low percentage of feathers and stems that add weight and bulk.
So depending on what types of activities you plan on doing and your climate the fill count vary.
For Snowboarding or Skiing etc go with 650 Fill Count Jacket
For extreme warmth and extreme conditions go with 800 – 900 Fill Count Jackets.
I use a Burton Snowboard Down Jacket 650 Fill Count for walking my dog in the winter in Chicago.
For going out, etc. I have a North Face Flash Down Jacket which has a 900 Fill count. Very lightweight and extremely warm!
Good Luck!
R