From the above test, it looks like GTX has practically the same resistance for water flow as PA. In real loop, with 200-300l/h water flow, the difference in flow would be hardly noticeable.
Also, we can extrapolate that GTX matches PA's performance with ~1000rpm fans and has higher performance with more powerful fans.

If the GTX doesn't need to be cleaned, I'd choose it any time over PA. Really, PA is very contaminating and at the same time hard to clean. After two months of use, the water turned into ugly milk, although i cleaned the rad before use with hot water. Everything inside including water block and res. was covered with some deposit. Luckily it was easy to remove, i found the plain salt works the best.
I used just plain distilled water with Tygon Silver tubing.
I have another loop with BI Xtreme (not GTX), the water had no signs of contamination after half a year of continuous use. I didn't even cleaned it before using.

Besides, from the aesthetic point, GTX looks better. PA has cheap painting which easily come out. I made a mistake and tried to clean the remains of gasket's glue with alcohol. The glue gone together with painting.