i can tell you for sure that it's not difference in level of difficulty. I got destroyed in PT 51(especially LR sec 3. I even remember the section number: that's how hard i got rekt), while doing much better on other ones.
One interesting approach I have heard is to take the PTs out of order. For example, 51, 61, 71, 52, 62, 72, etc, etc. That will give you exposure to the newer tests and their differences without burning through them.