I don't know squat about WC jr.
But I do know that Pritchard has a compulsive personality and drives himself even harder than many other NBA players. He gets the absolute most out of what he has. I remember reading how he would continue to practice dribbling a basketball even after his hands started bleeding from the repetitions.
That may be a point of separation. But then again, Clayton is 2 inches taller (6'3" versus 6'1") and based solely upon a single highlight video, Clayton may be a bit more athletic.
Pritchard is heavier by about 10 pounds, which might help with driving the basketball in the NBA. But then again, the Celtics' conditioning program might allow Clayton to bulk up.
Neither of them seem to be excessively quick or fast for NBA players.