This is an interesting question. I use Hoya Pro1 and B&W CPLs. I've also rented some Cokins just to test. I've seen a slight difference in the amount of polorizing effects between the Hoya and B&W, and a considerable difference between those two and Cokin. The Cokin gave me a much greater and deeper polorizing effect.
However, I not only look for the CPL effects, but what they do to IQ in general. While the Cokins gave richer results, what they did to sharpness and detail was totally unacceptable.
I would like a little deeper results from the Hoya and B&W, but now I'm wondering if thicker polorizing coating (not just quality of glass) also means less detail?
Some day, I hope to get Lee or some other high-dollar CPL.