In reply to jwa:
> Forget about fit and details
If the fabric is
identical between two items, then the
fabric will behave
identically. The difficulty comes in determining whether two fabrics are actually identical; as mentioned above, there are many, many variants of the big name brand fabrics; Polartec PowerDry, for instance, may appear to be one fabric, but it's actually a whole range of fabrics of different weight, densities, surface finishes, inner face structures, etc. Ditto ThermalPro, some of which is the fluffy 'bearskin' stuff, some of which has a dense, cropped face. Polartec have detailed fabric products codes, but they're a bit arcane; the only company I know that quote the specific fabric spec for garments is Marmot.
You mentioned 'keeping dry'. It's interesting to note that Gore insist they test all items made from Gore-tex, to ensure the design is adequate (and they have design rules they give to manufacturers). This is a good indication that design is just as important as fabric when it comes to garment performance.