Is down or synthetic insulation better for an expedition parka?

It depends on your typical conditions. Down (especially water-resistant treated down like DownTek or DriDown) offers the best warmth-to-weight ratio and packability, making it ideal for cold, dry climates like the Rockies, interior British Columbia, or Japan. Synthetic insulation (PrimaLoft, Coreloft) retains warmth when damp and dries faster, making it better for wet coastal climates like the Pacific Northwest or coastal Norway. Hybrid insulation, which places down at the core and synthetic in moisture-prone areas, offers the most balanced performance across variable conditions. If you're unsure, hybrid is the safest choice.