MCH and DRK were immediately viable for savage. People didn't even need much time to familiarize themselves with those classes. If AST were viable for savage, we'd see a lot of them in raiding by now. Since most if not all A4S clear groups still use WHM/SCH combo, I guess its safe to assume AST are still not quite viable yet. Lvl the class at your own risk but if I were you, I'd try to be on the safe side and level WHM instead.

I don't know exactly why raid groups are not using AST more. As someone who has played WHM and AST at lvl 60, I could say that WHM's divine seal is much better than synastry. WHM's medica II has a 5-yalm bigger range than aspected helios. WHM brings 30 MND buffs to both healers which translates to slightly more dps/healing.