Six Flags is not as good as Kings Dominion.
But first off, when are you going?
In my opinion, you can ride Kings Dominion coasters all day, and never get tired. When I am at Six Flags, I am asking to leave early. Kings Dominion also has a better up keep. The park has a better aroma. The only reason to go to Great Adventure would be to ride Kingda Ka, El Toro, and Nitro. Kings Dominion has Dominator, Flight of Fear, Volcano, Back Lot Stunt Coaster, (I love that ride) and of course, Intimidator305.
Think about it.