Since I have a new windshield seal from Corvette America (sponge rubber) and one from weatherstripspecial, I compared them and found differences similar to what Steve describes. The Corvette America strip is almost identical to the factory strip in looks and feel. It is softer, a glossy black, and has the thin rubber coating over sponge core rubber like the original. The weatherstripspecial piece is stiffer, and feels like a more solid piece but with the hollow core to give it some flex. It is also a dull black color and several shades lighter, so it won't look exactly like original. The corners and ends however looked identical based on a 60 sec or so comparison.
However I noticed more small flaws at the corners and seams in the Corvette America piece that are probably normal for that type of piece and were probably there in the original as well. Since I already know what the sponge rubber will look like after years in the sun (chunks missing, dry and crumbly and leaks like crazy), I think I'll keep the weatherstripspecial piece and begin a long term test. I'll save $70 for the time being, but will keep in mind that most often you get what you pay for, here's to hoping technology has improved both the quality of the part and made it cheaper to produce.
Maybe I'll remember to update this in a couple of years.