It really depends where and how yer going, what you're doing, and which bag is the better value/available kickdown when you need to acquire. If you're facing a lot of precipitation, I would suggest synthetic. But if you seek a small pack (I would also highly recommend carrying the sleeping bag in a waterproof compression sack), quality down has more warmth per weight/mass.
I got a near-new 0 degree, 2 lb Marmot down bag that compressed to the size of a basketball for $50 on Craigslist. I never had an issue with loose feathers despite beating the shit out of it. The synthetic outer fabric repelled water, but when sleep structures failed (bivvy losing waterproofness in Alaska at 3 AM, tarp on a 53' in a WY snowstorm, etc), misery resulted. Also, the down compresses with use, so you have to occasionally wash it with Woolite in a front-loading washer to retain full warmth. You can keep a synthetic bag dirty, or at least use cheap regular soap on it.
Current bag is synthetic cuz that was what was kicked down when my gear got stolen. 25 degrees and twice the mass.