Synthetic VS Down
This is a topic that has been covered extensively and there are a million write ups explaining the differences and pros and cons of different types of insulation. For the purpose of this write up I will give a broad overview of Down and Synthetic insulation.
Synthetic insulation is generally heavier and will not compress as small as down. It also tends to pack down over time and lose loft. Loft equals warmth. On the plus side, synthetic insulation is generally cheaper than down and will hold loft and provide some warmth when wet. It also tends to dry faster than down. Down insulation is lighter and will compress extremely small. The downside is it is more expensive and will not insulate when it gets wet. It also takes longer to dry out. Marmot uses Hydrophobic down or synthetic insulation in most of their bags with a couple of models using both synthetic and Down. Hydrophobic treatment is a method of covering down with a waterproofing agent that helps it repel water. This along with mixing in a small amount of synthetic insulation gives users the best of both worlds in one bag.
Sleeping Bag Care
Any sleeping bag whether synthetic or down needs to be properly cared for to get the best performance and longest life. There are two main things you can do to care for your new sleeping bag. Number one, never store your bag in its stuff sack. This is especially damaging to synthetic bags but is also not good for down. By storing your sleeping bag in the stuff sack, you hold the insulation in a compacted state which can crease and pack the insulation over time. This will make it harder for the bag to fluff back up once it is out of the stuff sack and achieve its full loft, and again, loft equals warmth. Most bags come with a small stiff sack and larger storage bag. Use that storage bag!
Second, make a habit of regularly washing your sleeping bag. Depending on how you use your bag you should be washing it after every 15-20 uses. You can always have a bag professionally cleaned but if you have a washer without a central agitator you can just throw it in the washer at home. I would recommend instead of detergent to use a wash designed for technical fabrics like Nik Wax Tech Wash Or Down Direct and Down Proof. This will clean your bag and restore the DWR and Hydrophobic properties that you are paying for. Wash in cold water on the gentle cycle and make sure your washer and dryer have enough room for the bag to fully wash. Sometimes your best bet is a large industrial machine at a laundromat. Dry your bag on the lowest heat possible and check every so often to make sure the bag is not getting too hot. When your bag is almost fully dry, throw a couple of tennis balls in the dryer to help fluff the insulation. When your done I recommend leaving your bag laid out overnight to make sure it is fully dry before storing.