What are your priorities?

First pick an insulating material:
Down (quality down)
- Best warm/weight/size ratio
- Useless when wet (However newer upmarket ones have water resistant down)
- When stored properly it will last a very long time.

Synthetic:
- Less expensive
- Not as useless in the wet
- Doesn’t last as long

Second, choose size/cut:
- Mummy style are lighter and smaller to pack. If size and weight is a big priority, choose one with the narrowest and shortest you can comfortably lie in.
- Rectangular can be more comfortable.

Third, choose shell material:
- Light is generally less durable
- Water-resistant/waterproof shell materials are available, at a price. Both money and size/weight wise.

Personally I like the size/weight ratio of downbags and there great durability. Currently still using one from 1992, some cold spots have developt. Still need to think if I want to refill those pockets or get a new bag.

The Beaume and snugpak you linked to. Could not find what the -25 rating is. Comfort or extreme and tested to which standard. Might be the convertion between fahrenheit and celcius, but the snugpak look like a much less warm bag then the Beaume. (a quality down bag would weight about half, but 3x as expensive)