They really can't do anything to each other physically (since they both have great durability and malleable bodies).
However, if Warlock is feeling evil, he wins by feeding on PM.
DEL
Delta1938True King of House of El
I say Plastic Man, but that's because Plastic Man is surprisingly powerful and the one storyline I read with Warlock, he didn't impress me at all. I could be completely wrong about Warlock.