I need a Duvet for my comforter. What is the best material to get, for the cold months?

My choices are cotton, flannel or micro suede. How much should I be willing to pay for a King? Any brands better than others?

I like flannel for warmth in the winter and cotton in the summer. Micro suede is better for blankets, IMHO. With flannel you need to take some extra care when washing to prevent piling. I was mine inside out, in cold water and on the gentle cycle. I put a tennis ball in the dryer with it so it dries evenly and doesn't ball up. I think the one I have was like $80 with a 20% discount coupon and you can find them at Macy's, Bed Bath & Beyond, Anna's Linens. Lauren, Waverly, Dan River all make durable ones.