Great point Aron, I am using a Marmot Hydrogen and its a little on the light side...but in my bivy with my jacket on its toasty. Nice to only carry a bag that's only 1lb. 9oz...
Down loses alot more insulative value when wet than synthetic does. A good example is that when your down bag gets wet it will lose quite a bit of loft. Also down bags are a pain to dry in my experience
Synthetic bags like the slick bag will keep you warm when wet. Thats the purpose of them. I have never done it, but have heard of people getting in wet and drying out during their sleep.
wearing clothing to bed is a great option / but if you are high in elevation or up north you may wish you had the warmer bag / i t is easier to remedy being to hot than to cold ( carry a marmot lithium 0 it compresses real small and is super lite and not bad to sleep on top of if it is to hot )
I agree bearguide and this really depends on each persons hunting style, area and time of year.
In Colorado I'm usually hunting between 10,000 and 12,000 feet for a couple months out of the year and haven't used a bag bellow 15 degrees. In the summer I run a 30 and even bring a quilt from time to time.
The big thing is weight savings and what you're willing to do to drop as much as you can.