Backcountry 0 degree down sleeping bag question

Isn’t the cosmic down 550 fill?

Between the two options you list, I think the Paria takes the clear edge.  Aggregate loft is greater, weight is like a pound lower, etc.  if you sleep that cold make sure you get the appropriate size and a good draft collar.  Two important things to note:

• Are you 100% sure your sleeping pad isn’t the issue?  Doesn’t how matter your bag is, if your pad’s r-value is too low for conditions you’re going to feel cold.

• If budget’s an issue, why not buy a down or apex over quilt and drape it over your current sleep system?  You can get fantastic used quilts via eBay or just buy a cheap new quilt.