I went through a lengthy discussion with the folks here on MM about sleeping bags about 4 months ago. I received some very valuable feed back from a lot of different people. However my search was mostly likely different than yours in as much as I was not concerned about the weight. I wanted, excess warmth, really large size, specific materials, I'll never use it during warm weather, and finally I'll never be back-packing the bag.
If you could give the folks some more specifics, you'll get a more accurate answer to your question, such as:
Do you want to back pack with this bag?
Do you want slick interior material to allow for more easy movement when your in the bag?
Do you sleep on your back. do you sleep on your side, do you sleep with your legs drawn up (fetal position)? Mummy bags are tapered so some folks can't stand to sleep in them because they don't accommodate some kinds of sleeping behaviors. Rectangular bags are roomer and allow for different kinds of sleeping behaviors but there are few rectangular bag designs to choice from and they a nearly always heavier per pound of insulation.
What is the worst case scenario for wind, rain and cold that you plan to encounter when your using this particular bag?
Will you alway be in a tent or do you ever sleep out at times because wind and rain can be as much an issue with warmth as just plain cold?
There are two type of insulation materials, down and synthetics. Both are well liked by a lot of different folks. Down still gets the nod by most folks for being the warmest insulation per pound but if it ever gets wet, your in serious trouble, if it's cold, because your body heat won't dry down. Second, down is more expensive pre pound than synthetics. Synthetics are warm, some claim as warm as down, if you get a heavy enough bag they'll do to -40 degrees. They claim you can sleep in some kinds of soaking wet synthetics and they will warm up and dry out from your body heat.
I would recommend you keep asking question here on MM and then do a lot of reading on various subjects like: down vs synthetic sleeping bags, compare the reviews on different kinds of synthetic insulations (they are not all the same), read the issues associate with mummy bags vs rectangular bags. Read up on how to store the bag when your not using it, to keep the insulating material fresh and functional.
Also, the ground pad and a bivy (cover) are every bit as much a part of getting a warm sleeping bag system as the bag itself, actually, I've come to believe they may actually be more important, if your going to do it right.
I still haven't purchased a bag but I believe I'm going to give a Wiggy bag a try, they are pricey but not that much more than other highly regarded bags. You won't find Wiggy bags at the most retail outlet but they are easily bought over the internet. (Wiggy is made is in Grand Junction, CO as I recall) Big Agnes seems very popular and they have a lot of variety and are very well liked by a lot of users.
Another good place to look on the internet for opinions on good bags are the back-packers forums. Some of these back-packer types hate hunting but they are not short on knowledge about what's light, durable, and warm.
Finally, your specific uses will be unique to you, again, the more you can share with the guys on MM the better your information will be.
Best of luck.
DC
