In reply to dazmac: Not used the PHD one but have had a DAS for 10 years and used and reviewed a Mtn Equipment Citadel last winter -
http://www.ukclimbing.com/gear/review.php?id=4444 the Citadel might actually be slightly warmer but as noted in the review, it has a few features that make it less easy to use for actual belaying.
Out of interest, why do you want synthetic in particular? I would have thought high himalayan peaks would be a a place where an equally warm but lighter and more packable down jacket would work?