The problem with not pressing charges is this guy will continue to get away with this type of behaviour.
If he goes to court on the drugs only his lawyer will claim it's an unlawful detention and search, he will claim that the cops made up the whole assault story in order to violate his clients rights.... If your not there to testify to what happened then there is a good chance he will be "walking".
It's a tough choice to make, and of course yours to make, but not pressing charges will not prevent this guy from being pissed off at you for him losing his meth.