Hi-
I've used both with my son. I have the newer edition of TWTM which suggested OPGTTR, so I started with that. (I had looked at a friend's copy of PP and it seemed a bit confusing to me at first.:o)
I liked OPGTTR at first because I was just starting "lessons" with my son and because it told me what to say, I was reassured that I wasn't forgetting anything. But after a while, I felt like we could use some more practice and got a copy of PP and started with that at the beginning, basically using it as a review. My son loves PP! He really likes reading the lists.
So, I've continued to work our way through OPGTTR because we were so far through it, but if I were to do it again I'd probably just go with PP.
Good Luck!
Shannon