First of all you have to decide what time of the year are you'll be going and what temperatures you'll experience. Remember that a 20 degree bag will be bulkier and more expensive than a 40 degree bag. However, if you're planning fall trips to the Rockies, temperatures can reach the 30's at night so a 20 degree may be the bag of choice. The next question to answer is what style of bag to purchase. In the old days there were rectangular bags, and mummy bags.
Rectangular bags are roomy and very comfortable but are bulky.
Mummy bags pack up nice, and are very warm but when I sleep alone in my mummy bag, I feel like someone has taken my arms and duct taped them to my sides. I can't sleep a wink. When I zip my mummy together with my wife's it works okay, but she accuses me of stealing her room, which creates a whole different set of problems. Some sleeping bag companies are making hybrids that are a cross between mummies and rectangular bags. This may be a good compromise.