We did both and found that we preferred Exuma.
The property in Nassau is narrow and many rooms have view of adjacent properties and parking lots. We went there in January when a cold front moved through and the air and water temps were 64 F with high winds as in the water was blowing out of the pool. Watch the time of year. Nice beach and amenities.
Since the water was rougher we did not go out every day and stayed fairly close. The Bond wrecks are piles of debris and we did not see a whole lot of life on most dives. We did do a wreck whose name I cannot remember which was decent and did a wall dive that was cut short as the current switched direction on us. We did see plenty of reef sharks on a couple of dives (Runway I think is the site) at the surface and underwater.
As for Exuma, the diving was better. Our trip was July 2010 and I think they ironed out the kinks in the hotel - converted Four Seasons. More corals but not a lot of life aside from lionfish, a few eels, and a few fish but did do Angelfish Bluehole and a wreck. Quite alot of sand but it is the Bahamas. All dives around Nassau and Exuma were from moored boats i.e. no drift with pickup. There was a scheduled dive to Long Ilsand from Exuma for a wall and wreck dive but was cancelled due to a lightning/thunderstorm that day.
As for the dive ops at Sandals, I think it's pretty much standardized across their locations. Both ops are safety conscientious, good boats, professional, and decent gear (we bring all our own gear).
They are more geared towards honeymoon or non-hardcore divers I think but we have got good dives especially in St. Lucia.
The do keep to schedules as the AM dives are usually for certified divers, the afternoon dives are for resort course and they use the same boats for snorkel tours and in Nassau to ferry people to the private island.
Be prepared for a checkout dive unless you have recent log book entries or have dives saved on your computer. The checkout (pool) may happen anyway.
However, I find for me that the diving is too regimented. I like to linger, explore and if I have my camera it becomes more of an issue. So much so that we went with a much more expensive dive op outside the hotel where we did the bluehole and wreck after a few days with Sandals.
If you plan only a few dives, Sandals ops will do the trick.
I wrote a trip report for the Exuma trip and it is in the Bahamas forum. I also have a few Exuma diving pics in my gallery (sorry they are mixed in with all others but will have titles).
http://www.scubaboard.com/forums/bahamas/343141-exuma-trip-report-long.html
There are also numerous threads on SB regarding Sandals diving.
Overall we much preferred Exuma. The property was better, the beach was fantastic and the diving better than Nassau (New Providence) IMO.
