First of all congrats!!
Secondly, it is probably dependent on what type of subjects you like to shoot. Turks and Caicos is well known for a lot of shark diving (presuming you are going anytime of the year besides the 6 weeks in spring when they use the boat to snorkel with humpbacks, which is pretty darn awsome two).
I have been on the TCI Aggressor, but not the Utila, and I was very happy. If you are a macro shooter go in the mid summer to shoot yellowheaded jawfish with eggs! (I believe my friend won the first prize in amature with this type of shot). There are also secretary blennies, sailfin blennies, and all the usual carribean subjects. The crew is great and the photo pro is a nice guy.
I have been on the Belize and TCI Aggressors before (and have probably 1000 dives in cayman)...and all though I havent yet to visit Utila...I think that TCI aggressor takes the cake as far as those carribbean liveaboard option. Try sneaking on in June with the Jim Church School of Underwater Photography for an added bonus if theyll let you!
Hope this helps, congrats again
Since you have been to Roatan, I would suggest T&C. I believe almost half the diving the Utila Aggressor does is around the island of Roatan and you've probably done most of those sites. It looks like they do most of the north side stuff that Anthony's Key does, including Mary's Place. The other diving at Utila is of interest to me, but doing all the same sites the day boats do on Roatan makes me less interested in the Roatan portion of the trip.
I've been to T&C twice and I love it. Great coral health and both times I saw reef sharks on most every dive. Compared to your Cayman trip, the reefs are more similar to what is found on Little Cayman; great shape, very fishy, not much or no current.
I highly recommend T&C to anyone looking at a dive trip in the Caribbean.
Also, Im in TCI right now diving with dive provo. Little choppy out there but you are right...the coral health is very good for Caribbean and cannot say no to the guys in the grey suits patrolling the wall!