down is sometimes lighter than synthetic, and with proper care it can last you more than 20 years. with heavy use the warmth of a synthetic bag can break down after 5 or 6 years.
i would also get a silk sleeping bag liner which will add breathable warmth and also help keep the inside of your bag clean, especially after a few days of backpacking. :D
and if you are REALLY cold (or if you are using a down sleeping bag in freezing and wet conditions) you should get an overbag to add warmth and draw the condensation layer away from the down. is this why you want a synthetic bag?