And remember, you can always wear long underwear and wool socks if it's too cool, and unzip your bag if it's too hot. The rap on down has always been that it loses its loft and insulating properties when wet, and thus is not a good choice for wet Western Washington. However, any bag is uncomfortable when wet, so most hikers carry their bags in coated nylon stuff sacks. Furthermore, down bags compress into such tiny bundles they can easily be kept inside the backpack. Furthermore, down bags are significantly lighter than synthetic bags.
Synthetic insulations retain loft when wet, dry quickly and are less expensive than down.
Lite Loft by 3M is considered by many to be the warmest, ounce for ounce, but other good ones include Polarguard and Primaloft, both now rated above DuPont's older Quallofil and Hollofil. The shape of the bag largely depends on its temperature ratings and the way you sleep. Mummy bags provide a tight fit for less carrying weight and more heat retention. Tapered bags give you a bit more room to toss and turn, but also weigh more. Rectangular bags are bulky, but great for summer camping, especially for nocturnal acrobats, and can be unzipped for use as a comforter.