John,
Both are great prices.
I would ask you to think about the the conditions in which you will be using your bag.
Are you going to be in a lot of wet weather?
If you are, the synthetic bag does have the advantage over the down bag in keeping you warm, should your bag get wet.
The negative to a synthetic bag is that is does not compress as small as a down bag.
Warmth in wet conditions vs. compressability.
I live on the West Coast, so I do not know if the humidity that you will face will cause a collapse in your down loft and rendering the down bag less warm.
Something else that you might consider, rather than a bag liner, how about using the jacket/clothing you have to layer over yourself inside your sleeping bag to provide additional warmth?
This gives you the advantage of duel usage: warm clothing for standing around in camp and to add warmth inside your sleeping bag vs. a liner which can only be used while sleeping.
As a side note: Given that sleeping bags are one of the most expensive items that a light weight backpacker will invest in, would you want to spend more money (if it is in your budget/means to do so) on a higher down fill bag that would be lighter? I would hate to see you spend $100 on a bag, only to realize later that you wish you had saved that money and bought a more expensive and lighter bag that would serve your light weight needs for years and years to come.
My 1st bag was a Sierra Designs Rosa (women's bag) that was on sale for $100 and it served me well before I learned about light weight backpacking.
Now my Sierra Designs bag sits unused and I have a 1 lb Marmot Atom 40 F bag and a 2 lb 3 oz Marmot Helium EQ 15 F bag.
Hope that this helps you out and good luck on your trip!
One day, I hope to hike the AT too.
-Tony