I have had pretty good luck over the years with Sierra design synthetic bags. Ive got 6 of them, 2 of which are 25 years old (I have never had a reason to purchase anything different). I personally have Never owned a down bag but have been tempted (Washington state resident) but just can't do it as I spend way too much time in moist if not wet conditions. I also own a few REI synthetic bags that have actually been very good bags and are super affordable. If you are really concerned about the ability to retain heat, keep in mind a really good pad (assuming you're on the ground) is worth it's weight in gold. A really good pad can be worth 10-15deg. Of course, depending on what you plan on using your sleep system for will determine how much money you will spend. It sounds as if your looking for a system for backpacking? Keep in mind the possibility of going lighter/cooler rating for a bag and pad given a sleep ware clothing package that you may sleep in (wool, down puffy shirt and pants, fleece base layer etc.). Just a thought.
Good luck in your search