It depends. Giratina boasts more defensive stats then Giratina-o, but Giratina-o boasts more offensive stats then Giratina.Giratina or Giratina-O? What sets do you prefer for each?
For the sets, it depends once again. I personally like RestTalk for Giratina and probably an offensive Calm Mind for Giratina-o, though I've never used it yet so I don't know.