I like synthetic insulation systems, as they are less expensive, not as susceptible to moisture as down, and in this day and age can be made essentially at the same weight point as the down counterparts (e.g., Marmot Elite sleeping bag series). The only problem with the synthetics is that it doesn't compress well enough and eat a substantial chunk of the backpack space.
As technology keeps getting developed, the last statement may not be as accurate today as it was a few years ago. Are there any synthetic sleeping bags in 20F and 0F categories that compress well. My reference points are Marmot Trestles Elite 20 (an older, not an Eco version), and Trestles Elite 0F (also non-Eco version). I am happy with both until I start packing them.
Hey there @Dmitry! I have the issue of being allergic to down so I did quite a bit of research into synthetic bags, I have a Nemo forte 20 degree bag that is synthetic, ended up having to by a sea to summit compression sac for it but anyways it compresses down nicely and fits into the sleeping bag compartment of my pack really nicely! I specifically have the women’s bag but my partner has the men’s tempo 20 and his is just as good for compressing!
Despite the advances in technology, synthetic today is not going to be able to beat the warmth to weight ratio of down or the ability to pack down small. While the initial cost of synthetic is lower than down, it has a much shorter lifespan. A good down bag will last a lifetime if well cared for. Technology has also advanced with down and most have treatments to better manage wet conditions and the ability to resistant absorbing moisture. The days of being paranoid of getting your down bag anywhere near anything wet are gone. Unless you are allergic to down or expect to be in very damp conditions, you might want to reconsider your choice of bag.
Unfortunately bags just don't compress down well when synthetic, but if you're open to quilts, you'll find they take up much less space. I own several synthetic quilts all by Enlightened Equipment. I love quilts.