We've done one dive trip to St. Lucia and four or five to Cayman Brac, plus diving off Brac on the Cayman Aggressor. As you can tell - we much prefer Brac.
St. Lucia is beautiful, it is one of the prettiest Caribbean islands we have visited. The jungle, hills, mountains, and the sea are gorgeous. But I would NOT stay there without AC, it is hot and very humid and the mosquitos are ferocious!
We tried to go off and do some exploring on our own while on St. Lucia and it was a bit scary and we encountered some unfriendly folks! After that we stayed on the resort property and only did some guided tour excursions.
We had lovely diving in St. Lucia but it was mostly small stuff - a good environment for macro is the best way to describe it, but I did see a flying gunard in St. Lucia and that's not something you come across very often - and the coral reefs were lovely.
I really like Cayman Brac, it is a quiet and rural area and undeveloped but it is still part of Cayman, which is a first world country and I am confident that the air conditioning will work well! It is also a beautiful spot, unlike many Caribbean islands - it is not just flat and sandy with some palm trees. The Brac is a gorgeous, a wild bluff that is riddled with caves. There are beaches and wetlands and bird sanctuaries and a Booby preserve.
The locals are friendly and you can explore all over the island without worry. We usually rent a car for one day to drive around and explore. You can also hike or ride bikes but it's hot! You won't have to exercise any food and water precautions on Brac or worry about mosquito-borne diseases.
The reefs are beautiful and you are very likely to see some bigger animals like turtles, eagle rays, sting rays, eels, and loads of friendly groupers - and sometimes a goliath grouper. You probably won't see many sharks but there are usually some nurse sharks and reef sharks around. The Russian Frigate (renamed the Tibbetts) is a great wreck dive. If the weather is good the dive op will take a morning boat trip over to Little Cayman for an extra price (not bad) so you may also have a chance to dive the fabulous Bloody Bay Wall.
I loved our trip to St. Lucia and I am glad that we went there but I like Brac better. Below are some videos and links that I have collected about Brac that you may find of interest, but I am sure that you can also find lots of lovely information about St. Lucia.
There's very little nightlife on Brac, so if that is something you enjoy then it may not be the best spot for you.
Have fun wherever you decide to go!
https://vimeo.com/165357805
https://youtu.be/NTcZso-Zg80
http://www.xray-mag.com/pdfs/articles/Travel_CaymanBrac_LawsonWood_60_locked.pdf
https://www.facebook.com/cayman.brac.diving/posts/874214046078664#
http://caribjournal.com/2015/09/15/journey-to-cayman-brac/
https://www.scubaboard.com/community/threads/shore-diving-on-cayman-brac.553128/