First of all, where are you? Based on your choices, I think Europe. Second, define "below zero" as in how much below and whether you mean C or F. Based on your choices, I am going to presume C (goes along with Europe).

The Millet is rated -10C (EIN comfort rating). Ignore the other two numbers. Trust me on this one. I found the Millet online from a Spanish company for about $404 Euros.

The Mammut is rated -25C, but ignore that; they say its the limit, so -10C is probably the comfort rating. The Mammut weighs more than 1 kilo more than the Millet. I see it online for 300 Euros.

Between these two, my choice would be a Millet, but you should look at Valandre as well-very highly rated, mostly sold in Europe. Other choices would be US brands such as Marmot (I have one), Feathered Friends, Western Mountaineering, or MacPac (a NZ brand, I have one).