Well I hate to be different but here I am.  

First keep in mind an all season comfortable bag is a myth unless its modular/layered.  Summer bags are easy.  Its winter that's a challenge.  Finding a good winter bag is a process where you end up with a bunch of bags you don't use because those temperature ratings mean almost nothing unless you are a penguin.  

From experience, here's some suggestions that may shorten the process.  Look at the construction rather than the brand.  That's the type, how much fill and what type of fill, outer shell, and design.  I'll give you an example of design, my favorite bag is a mummy, thick fill, waterproof shell and get this synthetic fleece at the neck and toes inside the bag.  I love down but man is it a PITA when it gets wet and if you heat, tent or camper, you are going to get moisture.  

Touched on in this thread, on the ground or on a cot, insulation underneath is very important because as you lay on ANY bag, the fill will compact and there will be heat loss.  The ground can suck the heat out of your bones and a cot with air underneath isn't any better.  In cold, I prefer a two layer approach, two ground pads with at a minimum one layer a dense foam like the Thermorest Ridgerest, IMHO still the best ground pad ever designed.  The one closest to your body, can be more for comfort just keep in mind the more air the more heat loss.  

Last but not least is you need to keep in mind to get the benefits of a bag fill, it has to have heat.  That's from your body. I've slept cold nights more comfortably in my underwear than fully clothed.  Same rules apply as outter wear, cotton kills.  Bag adders I like, synthetic long johns and fleece liners.  

The only way you ever find out how a bag works, any bag from any manufacturer, is get out there and use it in the cold.  Before you buy a new bag, its best to try the liner, bag liner, synthetic long johns, etc.  

Notes heat can be added via a hotwater bottle and no it doesn't have to be a Naglene full of pee. Tent camping, smaller is better so your heat source can be more effective.  Fav setup a 7x7 low profile tent with Heater buddy and packing no more than a two man pack tent with ECO candle lantern.  I've literally stayed warmer in these than guys in a massive wall tent with woodstoves.  

BTW, I've moved up in the world.  I now have a truck camper that can go anywhere a truck can go with 20,000 btu heater and a 6" thick queen-sized bed.  I haven't found my limit on this yet but God willing, I will.  Everything has a limit and you start adding stuff.  The only exception is a boat because they're in water and once the water freezes the boat doesn't go.

TJ