As it stands, no classes have this kind of "choice" outside of tanks. That is a problem. Tanks shouldn't have a choice either.

Suggestion 1: Take slaying away from tanks. Make accessories class specific. Cut the vit on fending in half, replace the lost points with str. This will do two things. End the argument of str vs vit, and allow better tuning of bosses and of classes. It would be a lot easier to bring all tanks within 150 dps of each other.

Suggestion 2: Make parry actually useful and add def and mgc def to fending accessories to encourage tanks to use them, this would still run the risk of tanks doing exactly what they do now. So in turn se would make bosses hit harder and in turn bosses would more easily one shot everyone else.