PAUL UDOVIC wrote an answer about Arc'teryx Gamma MX Softshell Jacket - Men's on July 2, 2014
Fortius is a bit more breathable and is slightly warmer than the old Polartec PS. This is not based on any scientific testing but just regular usage of both versions of the jacket. I did find powersheild to cut the wind a bit better. lastly I find the fortius fabric seems to resist fading and the discolouration at he cuffs and hems.