OK, so my current bag is a Mountain Hardwear Phantom 32 which has served me well for summer backpacking in Colorado. I've camped in temps to around freezing and been happy with it. I'm looking for a bag that I can used for fall/winter backpacking in Texas. The lowest expected temperature will be 15 degrees F (I don't plan to camp in temperatures below that for the time being).
Here are the contestants thus far:
Mountain Hardwear Phantom 15 (15 degrees rating)= 1 lb, 15 oz (listed weight)
Marmot Helium (15 degrees rating) = 1 lb, 15 oz (listed weight)
Western Mountaineering Versalite (10 degrees rating) = 2 lb (listed weight)
Western Mountaineering Alpinlite (20 degrees rating) = 1lb, 15 oz (listed weight)
Western Mountaineering Ultralite (20 degrees rating) = 1 lb, 13 oz (on website) or 1 lb, 10 oz (on backcountry.com)
I should mention that I will be likely wearing a down jacket such as the Mountain Hardwear Phantom in my bag, along with a fleece hat and gloves. I should also mention that I am around 5'7" and 140 lbs. I prefer a comfortable fit–the Mountain Hardwear Phantom 32 with 60" shoulder girth fits me perfectly, although I'm not sure if I would need extra room for the down jacket. Finally, let me say that I used a Mountain Hardwear Phantom 15 LONG last year in temps to around 20 degrees in 60-70 mph winds (more likely 0-10 degrees with wind chill) and was somewhat cold. This is with the down jacket, too. I'm guessing that the extra length might have been the culprit.
I need some feedback on whether the listed weights above are realistic, as I will likely choose the lightest bag for the warmth that I require. Also, are the girth measurements listed on the manufacturer's websites accurate? The Marmot Helium seems a bit big at 66" shoulder and the WM Ultralite seems a bit narrow at 59". 60" or 62" sounds about right for me.

