OK -- my two cents! I have worked on the T&C Aggressor for a few years, and I can tell you that they only intentionally dive off of Grand Turks twice a year (on their way to the whale charters), unless the weather is really bad and coming from the West (that may happen only a few times a year). The VAST majority of the time, the T&C Aggressor dives off of Providenciales, West Caicos and French Cay. The diving is great no matter if you like Reef sharks and Eagle Rays or gaudy clown crabs and cleaning shrimp. Although there are better spots in the Caribbean for individual types of animals, In my opinion (having dived most of the C. -- including utila) I believe the Turks & Caicos Islands offer the best overall diving in the Caribbean! Off of each of the three islands you tend to see slightly different marine life. In my opinion, Providenciales offers great macro subjects (my favorite stuff), French Cay is a great place to find Wide Angle stuff like sharks, Rays and beautiful reef scenes, and West Caicos offers the best of both macro and WA. All three islands are terrific!
I have also lived in Utila for short while (land based) and although the visibility while I was there just about the best I have ever seen, I found the fish life a little disappointing. The Sea Mounds surrounding the islands are the most diverse and of course, in Utila you always have the possibility of spotting a whale shark.
The T&C Aggressor boat is very nice and quite comfortable however the Utila Aggressor is less then a year old now and while I haven't been onboard the Utila Aggressor, I have heard great things about the new boat!
I hope this helps, and if you have any specific questions, please feel free to Message me. I am also a photographer so I would be happy to discuss specifics with you if you like!
Happy Diving!
Melissa