I have no personal experience with Pertex or the PERFORMANCE of either fabric, but I DID read this website the other day, and they HAVE compared the two fabrics (about 3/4 of the way down the page on my screen):
http://www.mountainlaureldesigns.com/fabric.php
Also, if you're still interested in Pertex, this online vendor has some:
http://www.justmakeit.com/fabrics/water_repell/index.html
I looked at Pertex for the shell fabric for a down quilt I'm going to be making. But at $20/yd I just couldn't justify the cost. I went with Momentum, and then found that link comparing the two the other day. Boy am I glad I didn't shell out the extra cash!
Thru-Hiker.com has Momentum90 for $11.95/yd (personal experience, the purple is a great color, but then, Paul Nanian over at Thru-Hiker.com says that all the Momentum colors are fantastic):
http://www.thru-hiker.com/breathable-fabric.html
Hope this has been helpful…