Also, note the down rating, which has to do with the quality of down. I believe they compress a certain amount of down, and how much it covers/fills when expanded is the number it gets. So higher is definitely better.
Nothing, and I mean nothing is warmer than down. A good down bag can not be beat. A down bag has the unique ability to keep you cooler when it is hot and warmer when it is cold (not dramatically);
…however, down loses every single bit of its insulation properties when wet, and as above poster mentioned, compression and storage is an issue as well.
With that in mind, I prefer synthetic (especially for touring).
Downs # 1 enemy: Water
The # 1 most pervasive substance known to man: Water (maybe air, not sure)
As far as the bottom side, it varies by bag. North face, I believe, makes a bag with 2 temperature ratings. Just flip the bag over and it’s a warmer bag.
Also, some companies make bags with zero insulation on the bottom, to save weight and space. You just slide your sleeping pad in the slot on the bottom.
They have some awesome synthetics out that are so durable and retain such a high percentage of their insulating properties that a down bag for touring just doesn’t make sense to me.
But I do love me some down.