What kind of riding do you do? I've always understood the Psylo to be more trail-bike/real rough XC oriented than the Duke. I also believe that it weighs a bit more than the Duke. I have only ridden the Duke, so I can't compare it to the Psylo from my own riding experience. Its only a ten dollar difference and the Duke is newer, but its up to you. Good luck!
-Moab
P.S. - I currently ride a Manitou, but have ridden many friends' bikes with Marzocchi forks and have always been surprised at how nice they are. Might be worth looking at if there is something in your price range.