The theory is that the material doesn't provide insulation, the trapped dead air space does. Therefore the type of material shouldn't matter much. Some materials are better conductors/worse insulators than others (copper wiring vs aluminum wiring), but I'm not convinced that one fabric is noticeably better or worse than another.

My opinion, to be taken with as many grains of salt as you like, is that the dead air space matters most with any fabric insulation. The big difference will be in how long the fabric (filament, poly, down, whatever) is able to maintain it's dead air aspace, aka loft.