I'll say this much: I have a get a great deal on Marmot Sleeping bags.
For my 2013 PCT thru-hike I am therefore only really considering their bags.

The problem is this: The standard bag I read about PCT thru-hikers carrying is 20 degrees. Marmot makes 30 and 15 degree bags (plasma model) that are super light!
I don't really know what temperature bag to get. I've honestly never been aware of my bags rating before and I guess I've been lucky enough that it's worked. Will I cook in a 15 degree bag? How do I tell if I'm a cold or warm sleeper (I tend to get cold)
Will my long underwear be the key...

Thanks for the help, I don't want to carry the extra weight of a 15 degree bag because I'm afraid that a 30 won't fit my needs.