You absolutely want breakaway glass, since you can never guarantee real glass will break, let alone in the direction you want without endangering everyone with flying shards.
However, until it is swept up or melted, breakaway shards are still sharp and will fly - you need to try a few in order to show the actor exactly in what direction and wiht what force to drop it so nothing goes flying into the house or into the actors onstage - and no one gets injured until you have a chance to clean it up.
Have a blast! Breaking stuff onstage (deliberately) is fun!
