I liked gchem destroyer. I did very well on it (~80% correct after chad and KBB). In many of the challenging questions, if you REALLY do know the principles, it should not be difficult. If you do not, use it as a tool to learn them.
There are a few questions that introduce new ideas to you, so those are forgivable if you are going through it the first time.
Ochem seem to have slightly more of these completely new concepts thrown in that you do not see in Chad's or KBB, so you may get lower scores :X.
However, I have heard from numerous people that OCHEM Destroyer is ABOUT THE SAME difficulty as the real DAT, while genchem is much more difficult. So depending on what you are having trouble with, it may or may not be