Warmth
When comparing the two, down is much warmer than synthetic. Being warm is vital in camping. If you can't keep warm at night, you won't be able to get a good night's rest. My philosophy is to get a sleeping bag rated lower than the temperature you expect to be in. If you are expecting 0 degrees, maybe get a -10 degree bag. It leaves some room there in case it drops a little colder in the night and it's much easier unzipped your sleeping bag than to try bundling up. If you want warm (and compressibility), down is probably the way to go. Down feathers are the undercoat of ducks and geese that keep them warm. Therefore, it has it's own warming properties. Synthetic is also warm, but it takes a thicker synthetic to reach the warmth of a down bag. If you desire warmth, then you should go with down.
Compressibility
Another advantage I see to down is its compressibility. When it comes to backpacking, compressibility is vital next to weight, which is also an advantage to down. Down will compress very tightly and has very little weight to it, which is why it's preferred by many backpackers. My down bag fits in a small compartment in my backpack with no problems. If compressibility and saving room in your pack is your biggest concern, bear in mind that synthetic does compress down tightly as well, just not as small as down. Also remember that if size is your main concern, you can always use a compression sack to reduce the size of your bag (highly recommended). You can also put your bag in a stuff sack and strap it to the outside of your pack. If you are planning, though, to put your bag in your pack, you can compress down to about half the size of synthetic. 
Cleaning
Synthetic and down clean roughly the same and should be cleaned with care. I tend to be very gentle with some of my gear and sleeping bags are one of those. You really should hand wash a sleeping bag. Cleaning by hand ensures that you don't offset the filling or ruin the outside. All sleeping bags, in my opinion, should be air dried. Most bags have loops at the feet so you can hang them to let them dry. (Just make sure it's dry before setting out on your trip). Synthetic does have the advantage over down in this area. Synthetic will dry much faster than down and won't add much weight when wet. If you are trekking in damp climate or expect rain or moisture, you may want to go with synthetic. Down will hold in moisture and will get much heavier as where synthetic won't. I much prefer synthetic in winter, even though it takes up more room in my pack, because if it gets wet or soiled, I don't have to hike back out with an additional 4 pounds.
Comfort
I have come to believe that comfort is a personal preference. Some people are more comfortable in down and some are more comfortable in synthetic. Either way, you should try both to decide what you like. I recommend sleeping a few nights in each in your back yard or even inside to determine what you think is more comfortable. Comfort is important when camping and especially on multi-day treks. If you're not comfortable, you won't sleep well and will be miserable in the morning. Find what is more feasible for you before setting out. I personally think synthetic is more comfortable, but I have a buddy that swears by down. It's really up to you.
Overall, I think synthetic and down sleeping bags are equal. They both have their advantages. Your bag really depends on your preference, need, and what type of trip you will be on. Just be sure you try both before deciding (you may decide you like one that you wouldn't have though you would have). Comment below and let me know what you think.
