I've never been to Grenada but I just left Providenciales, Turks and Caicos yesterday. It was an awesome trip. I stayed at the Ocean Club West (not all-inclusive) and dove with Caicos Adventures, a solid, well run dive operation that does all of the best sites.
Turks and Caicos diving is a blast. The better dive sites are a 45 minute- 1 hour boat ride from Providenciales, but the seas were calm enough so that wasn't a problem for anyone on board. Dive sites are all pretty similar, they are made up of a sandy bottom dotted with coral heads leading to a wall. The walls crest pretty shallow, 35-60 feet at most sites, and according to the DM, drop 7000 feet. The terrain is spectacular in itself. The walls are coated with a good layer of sponge growth, including barrel sponges big enough for a diver to climb into. Huge schools of fish come to feed off the wall, everything from creole wrasse to horse-eye jacks to what my buddy thought could have been tuna (I wasn't sure). I really didn't expect such a concentration of life on an Atlantic dive.
For me, the best part of the trip were the pelagics. Now, granted, this wasn't the Galapagos, but it was still pretty impressive for an Atlantic/Caribbean dive. There were reef sharks on half of the dives, many of which weren't afraid to swim in for a close look. Nurse sharks were also around. On my last dive at French Cay (one several remote dive sites accessible as a day-trip from Providenciales), I saw two eagle rays, one of which must have been 7 feet across. Throughout the week, people saw a manta ray and dolphins (yes, underwater), but I was unlucky and saw neither. Conditions couldn't be easier, bathtub warm water and zero currents. I'm going to get a lot of flack for this, but I would compare this diving favorably to the East End of Grand Cayman.
The island itself is not the most dramatic. It's mostly flat and covered with little stunted trees, bushes and cacti. What Providenciales does have is Grace Bay, my all time favorite beach in the world.
The one thing to note is that this island is EXPENSIVE. But since you're going with all inclusive, I'm not sure that's gonna be an issue. Providenciales does have a Club Med and a Beaches. I'm not sure if they insist that you dive with their on-site dive operations, and I'm not sure how good they are.
Either way, happy travels.