I'd say level CNJ, THM, and ACN up for cross class skills first. Remember AST is going to be added with an expansion. In the MMO world, expansions are a time of sweeping changes to combat and class semantics. The current WHM and SCH may not be quite the same come 3.0. You may like them more, you may hate them more. Point being is that everything in 3.0 isn't set in stone. That's why I suggest any least working on the base classes in case they keep cross-class a thing. That way you are kinda prepared for the future. After that I'd recommend playing SCH. It's a bit more fun and more rewarding than WHM due to it being a jack-of-all-trades job and not just a healer.