To make it clear I'm not asking between all 3, I'm asking between the 2 of them, D/P and PT. Or if they have the same difficulty level, just say so.
PT > D/P
IMO PT is hard because there is the distortion world, the battle frontier, and elite 4 and champion have better Pokemon, as well as the gym leaders. But though more difficult, it is also a lot of fun too.