Ideally, Isolate is the best for PWO shakes. You want to limit fat intake to as minimal as possible in your PWO meal/shake due to that being a very susceptible time for protein uptake (and fat will only slow down that process).
Again, YMMV, I was using a blend for two years before I switched to Isolate, but I still use Blend on non-PWO meals. I also went over to the Lean n Fit isolate, this way if you are watching your caloric intake, you don't have to use milk because there is a good amount of fiber which thickens up with water use only (I wouldn't even TRY milk at this point).