If your goal is to save weight and maximize warmth, then I would go with the Ghost. It's smaller than the Specialist so you can stay warmer (less space for your body to heat). If your going to layer up, then get the Specialist. The extra width allows for layering up on colder nights, or for going into shoulder seasons. I find that the Ghost is large enough for me to put on some extra layers of clothing provided that they are thin: Patagonia Micro Puff Pants and Montbell Thermawrap Jacket (I'm skinny).
You can order a third strap on the Ghost, it only costs $5.00 extra.
The Ghost also has an extra 1/4 inch baffle height. More loft, warmer bag. I have 1 oz over fill on mine, seems more comfortable to me and maximizes loft while controlling slump (not a side sleeper, so not usually a problem). Over fill should also help the bag loft better with the heavier Epic fabric as opposed to the standard .85 fabric.
If your really intrested in the Ghost or Specialist you should call Nunatak, Tom has better info on these bags than I do. He's a really nice guy and very helpful. He answered all of my stupid questions without making me feel dumb (even though I am).