if you don't have enough info yet - or you'd like to get more confused - what about a bag liner and a regular bag? you've got a wide temp range.
You could go with a 45F down bag, which will pack really small, then add a liner when it get's colder - something like this: ALPS Mountaineering Fleece Sleeping Bag - Special Buy - REI.com (i've got a nylon shell thing with a fleece lining i put my regular sleeping back into. kind of a pain with two bags, but it's another option)

I'll do that backpacking, as i've got to share gear with my son. i'll use the 45F down and the liner, he'll take the 40F synthitic, or i'll use the 40F synth. with a liner, he'll take the 0F Polar Pod.

The liner packs super small, the 45F down is small. you get a nice air barrier in between.

note - if you don't already know - down is not warm when wet, synthetic is (supposedly) - best bet is keeping everything dry.