If you're really looking to go to -18C then you have no choice but to go with a down bag as that's what has the most insulation to weight value, and you did mention wanting to keep the weight down. The bad news is you can't do that for less than $400. Regardless of your temperature demands, personally, I would never settle for a synthetic bag. I have the Rab Neutrino Endurance 400 and absolutely love it. Yes it's got the Puretex treated down and in my view it's not a gimmick.