I would think that a PB is what you would want to use for pulled pork. It has plenty of internal fat that will produce moist meat, along with different muscles that will produce a nice variety of pulled pork.
As far as the cushion roast, I had to look it up and see what it looked like, and it really looks rather lean. I wouldn't want to make pulled pork, it would just be too dry.
difference in the two