I've been on the Cayman and T&C Aggressors and on a different LOB in Belize, but I have only done land-based diving in Roatan.
Of those 4 choices I like the diving best in Cayman and the T&Cs but both of those 2 destinations have pros and cons.
Cayman has beautiful reefs and healthy populations of sea life but not many sharks, but you should see a few sharks and you will probably have an opportunity to dive all 3 Cayman Islands. You will also see turtles and stingrays and (if you make the crossing) lots of friendly groupers and eagle rays. Many divers feel that Bloody Bay Wall off Little Cayman is some of the best diving in the Caribbean - so that is worth a trip!
And if you are interested in wreck diving their are some interesting wrecks around Cayman like the Kittiwake and the Tibbetts (a Russian Frigate). There is always the risk of weather issues that will prevent you from making the crossing but I am sure that the Captain will do his best to make sure that you have a good week of diving.
The Cayman Aggressor is a newer and larger boat so that might be nice. The toilet was inside the shower in our cabin on the T&C Aggressor 5 years ago. You might actually get a separate shower and toilet on the Cayman Aggressor!
The T&Cs have healthy walls and reefs and good sea life and a better chance of seeing lots of sharks and eagle rays, but the walls are prettier around Cayman IMO. The T&C reefs start a bit deeper so if air consumption is an issue of concern then that is a factor to consider. I can't recall any good wreck dive sites in the T&Cs but I may of forgotten something.
The T&Cs were hit hard by hurricanes last year and I don't know if the best sites are fully recovered yet so that is something to check - ask if they are regularly diving French Cay and West Caicos. If not, then I would give the T&Cs a pass and more time to recover.
On the other hand, you have a shorter window of opportunity to dive the T&Cs from a LOB because they divert to the Silver Banks from January to March for whale-watching. Since you are planning a trip later this year it might be a good opportunity to dive the the T&Cs - before the water gets cold and the boats relocate.
However, the land-based diving is very good around Cayman, because the reefs are close and you have short boat rides and shore diving is available. That may make the T&Cs the better choice for a LOB. In my opinion, the best way to dive T&Cs is from a LOB because the reefs are far from Provo and you will spend a large part of the day in small boats just to dive 2 tanks with land-based dive op.
Like I said, they both have pros and cons - so pick one for a trip this year and plan later for a trip to the other(s). And you may not want to limit yourself to the Caribbean Aggressors. We also did Aggressor trips to the Red Sea and Kona, HI and they were great, too! It's nice to have choices!