Been to Sandals Negril a few times.
Kids stayed at Beaches, down the road.
We visited em, very nice resort. I think there's a waterpark there too.
The Ocho Rios Sandals/Beaches is newer. Haven't been there.
The actual Ocho Rios float isn't that interesting, if that was a selling point for you.
Scuba diving is more developed around Negril. Included in Sandals package, don't know about Beaches. Good location to do our Advanced, or Rescue if you want to work that hard.
Can't go wrong either way.
