Ski_Adk
here's my take:
1 - primary consideration is rating. For true 3 season camping, if you are cold sleeper go for 15F, if a warm sleeper 25F. always wanna have a little more than think will need
2 - secondary consideration is comfort. i cannot over-recommend the Mt. Hdwr quantum system. is like having two bags in one, togther w/ the full double zip venting you WILL be able to teak the temp rating. Kelty footzip is nice but doesnt compare. so, go for a little lower temp rated Mt. Hdwr quantum bag, say 15-25F. anyone who says one bag cannot do 3 season comfortably has not has the pleaseure of getting acquainted w/ a Mt. Hdwr Q bag (i own two).
3 - weight is IMO secondary to size/packability - AND - more importantly, down is a better buy because you can smash the you know what out of it down to nothing and it will spring back time and again 4-ever, whereas if ya do that to synthetic it might last a year... also, fears of getting down wet are greatly exagerated unless you are a careless camper...
4 - BOTTOM LINE - spend a few extra bucks to get a Mt. Hdwr down quantum bag rated a little lower than what you were thinking and you will set for a long, long time.
and will thank me over & over,
Gris