go with og, the iberostar charges dive ops other than their "on site" dive op to use thier pier. haven't heard much good about the on site dive op, a german owned operaton. they seem to cater to folks doing "resort dives".
have also heard that the rooms aren't set up for divers, no place to hang gear and they don't want you using the balcony to do it. have also heard of other custumer no-service problems.
we stayed at the og in Sept. and really liked it, the food was really good, not the same thing day in and day out on the buffets and the reservation restaurants are excellent! the grounds and the rooms are beautiful, small balconies but enough room and no problem hanging gear. our room also had both a tub and a shower to hang stuff on and around.
Dive with Martin is aslo a good choice, great op. you will need to take a cab down to his shop to sign in and pay when you get there, $17 one way from the Grand and not much less from the Iberostar. by the time they get down to the Grand to pick you up for diving, it will be a little after 8:00a, but you will probably be the first to be dropped of, definatly before noon. we were getting back in plenty of time to wash off our gear that we didn't leave with him, take it back to the room and make lunch.