Have not been to Tobago, but my TCI trip was awesome. The diving in Grand Turk is excellent, but there isn't really much else to do there. Provo, on the other hand, is spread out, commercial, and the diving is more expensive because of the travel involved. The video link in my signature below was shot on this trip, might give you some idea of what to expect. I would recommend splitting the trip in half. Spend the first half on Grand Turk and stay at the Osprey Beach Hotel, dive with Blue Water Divers. Fly back to Provo for the second half and stay in the Grace Bay area and dive with Caicos Adventures. It's like two dive trips in one. We did see some Lionfish (see video), but the reefs seemed very health at the time.
Hope this helps