One of the most important pieces of gear is what you decide to sleep in. Especially when you are sleeping in it for months on end. The one unfortunate thing about sleeping bags are their rating system -- or lack thereof. Here in America, there is no standard way of rating a sleeping bag, and even if there were, the differences between people's circulation, comfort, etc. makes it difficult to really know how a sleeping bag will work for you.
The first and foremost thing you must do is test out your bag. Companies like REI and backcountry.com allow you to return used products if they don't work out, so it does make sense to purchase two sleeping bags, and see which you like most.
There are a few key factors to take into consideration: the varying temperatures of your trip, the conditions (super rainy, or bone dry), and lastly, how you personally sleep. I am a warm sleeper. I would say a hot sleeper. And because of the fact that we have been in the desert for the past month and a half, and will be in the mountains soon, I needed a bag that would keep me comfortable without overheating me, or letting me freeze.
Down is the choice for me. Down regulates temperature better, and with the Phantoms 800 fill power, keeps the bag lightweight (FYI, fill power has NOTHING to do with warmth. It has to do with the quality and weight of the down). Synthetic material is good for wet conditions when cmaping in a bivy, and the potential for the bag to get wet is high, but it weighs much more, and is much bulkier. Keep the down in a dry bag, and it will be good to go, take up less room in your pack, and be much lighter!
The Phantom weighs in a hair over 2 lbs. For a 15 degree bag, that's pretty darn good! The bottom is on the tighter side, but my legs aren't suffocated or squished in there, so am happy they cut down on the weight and slimmed it down a bit. As it goes up the bag, it widens out -- big time. There is plenty of space to move around and get cozy. The draft guard is an extra bit of down that covers the backside of the zipper preventing any air from hitting your body through the zipper, making for a comfortable sleep.
My one qualm with the bag is the mummy hood, it's a sliver. When zipped up all the way, your face is covered and only room for your eyes to peak out. This makes for a good cold weather bag, but gets a little in the way on warmer nights when it's not zipped up all the way. I would prefer a little more room for my head up top, but I'm sure I'll appreciate the extra warmthin the mountains to come.