Almost fifty years ago when I did my Canadian Infantry Winter Warfare training they issued me a bag that came in six pieces - four layers, a carry sack, and a rubber coffin shaped air mattress. That experience stuck with me and I still use what I learned during that training. First thing I learned about sleeping bags is I hate mummy bags. They are too narrow for me to be comfortable and it is difficult to store the clothes you are going to wear at the bottom of the bag to keep them warm. Second thing I learned is that those sock like liners for inside the bag end up getting all twisted and strangling you in the middle of the night. The third thing I learned is that those nice high air mattresses aren't that warm and if you move around you fall off of it. Finally, I learned that if you can't get a good nights sleep your trip becomes ugly and dangerous.
In the years since, I always bought bags I could turn over in and sleeping pads large enough to hold me. For short trips - a few days - down is the best, but in really cold temperatures down can pick up condensation and hold onto it, getting heavier and less insulating, which makes for a tough long trip. I have been known to put a flimsy synthetic bag inside a good down bag to prevent that. Synthetics will dry out relatively fast, especially if you can hang them over a tree branch or bush in a breeze at some time throughout the day - like when you are glassing for animals. Get a good quality bag you can stretch out in, and pull up above your head if necessary, and carry and extra layer or two to add to it. And make sure the bag has a well insulated zipper that doesn't bind when you are trying to get in or out of it. Fighting with a sleeping bag zipper in the middle of the night is one of life's more frustrating events.