Does anyone know how the Marmot Ion and Western Mountaineering Ultralite sleeping bags compare? Both are rated to 20F and are advertised as 1lb 13oz, while the Marmot is down on top with synthetic on the bottom and the Western Mountaineering is all down. Marmot claims the synthetic is better than down on the bottom because it doesn't compress as much... The Marmot is also cheaper and significantly roomier in the hips and ankles according to the specs. Do these companies have similar temperature standards? Are they likely equally warm or equally durable?