I've had Optimus Prime Petit and Seedorf for a while now, thoughts on them:
Petit is a monster, has a very powerful and accurate shot, covers the pitch well and never runs out of stamina. Passing is good, strength and tackling are great. He's a bit like Gullit with less of an attacking threat. Great presence on the pitch too.
Seedorf is less impactful, you have to try and get him into the game more, however his passing and shooting are exceptional, he's enjoyable to use but not as key to the team as Petit. I've used the Socrates for 10 games as a loan and I wouldn't say he's much better than Seedorf really.
Considering Petit is a box to box player, who is just 'good' at everything, he's great fun to use. I've scored some amazing long range shots with him.