Temperature ratings are meaningless! I have an older WM Super Antelope10-degree bag and I get cold at 20 degrees. My partner, has enough body fat and fuzz and a hot-shot metabolism and sleeps hot in a 40 degree bag in 20 degrees. It is all very individual. Also, the rating is based on the bag fully cinched up with just your nose hanging out, on a perfectly insulated sleeping pad. If you cannot sleep in this position, then you need a warmer than average bag.

Quantitative comparisons can be made in fill type (650,700, 750, etc down), fill weight, cover material, design and fit. I would never buy a bag not knowing the fill weight. Try to find charts that compare these. Call the manufacturer to get detailed design information (baffle type, draft collar, draft tube, etc).

Loft is a pretty relativistic measurment. Loft when absolutly new, never stuffed, right from the manufacturer? Loft after stuffed for 16hours? Loft after a year's use? Loft after three washings? Hard to say.

You need to try out several bags (borrow from friends) to see how the standard temperature rating feels for YOU. Or maybe the store would let you take it home two or three bags, sleep in them on your clean carpet once in the winter with no heat on in your house - then compare. (or buy from an supplier with liberal return policies).