In reply to jezzah:
In terms of warmth it's unlikely there'll be a huge amount of difference given the ratios of fill power:weight of down but ultimately I would say the Positron will be warmer.
The main difference will be in weight and durability. The infinity has a very lightweight outer (less durable) compared to the positron. If in your case it might not be used technically much then I would say positron.
I've had a neutrino endurance for years and that thing keeps going!