I have a down bag that has served me for the last 25yrs in North Dakota, Montana, Wyoming and Idaho. There are two requirements to making them last.
1. Do not store the bag in the compression sack. Instead buy a large plastic tote and when not in use store the bag in the tote.
2. Do not allow the bag to get wet. Pick a camp spot with good run off. Buy a good quality ground sheet for when you are camping. I use two, the first goes under the bag and the second acts as a tent unless the weather is really bad. When backpacking with the bag in the compression sack make sure the bag is also stored in a second water proof "dry sack" or use a trash can bag. These are also handy for acting as pillows.
Hope this answers your questions. I would recommend talking to the folks at a quality camping supply because you are looking for the best compromise betwee light weight and durability and they should be able to point you in the right direction. The folk at REI are generally very knowledgeable.
