That may be true, but just like mr.lol said, it doesn't matter if it breaks the Sub or not, because you will still not phaze it anyway. Sure, breaking Subs would be nice to force them to waste a turn and 25% HP setting one up again. However, the choice between Giratina-O and -A is purely up to team style. Giratina-O works better on offenseive teams, while Giratina-A is suited more for semi- and full stall teams. If you are that concerned about offensive pressure then keep Giratina-O, but Giratina-A's defenses are better for a team based on keeping hazards on the field as long as possible.
[22:11] Helias: FUCK YOU HAWKSTAR THAT'S NOT MY NAME