If i was spending good money on a new waterproof, i would choose Pertex over Goretex any day of the week.
Gortex is fine, when the water repellent coating works efficiently, but lose that repellency, and the fabric soaks up the moisture like a sponge, and it will take ages to dry, unlike Pertex.
Pertex is a more robust material than any garment made from the Gortex fabric, and dries quicker than just about any fabric i know of, and maintains its amazing wind proofing capabilities even when saturated with water.
Marketing i am sure has a lot to do with it, with many potential new customers unsure of the benefits of Pertex over Gortex.
All they see is an attractive jacket in a shop, and Gortex is a big name in the industry, so must be the best waterproof material out there.
It may have been twenty or so years ago, but the situation has moved on considerably since then.