Both good places for diving , either one will be nice clear warm water with lots of marine life. If you choose Grand Cayman and you want to get something to eat after the dive take a huge pile of money, 14 credit cards, and then be willing to cut off an arm to pay for a hamburger. They have a license to steel there and they smile while doing it.
The diving is very good in the Caymans but they are an expensive destination, especially since the $USD is only worth $0.80, but there are ways to mitigate the costs somewhat. When we are on Grand Cayman we stay at a nice, clean, small, family-owned inn that is on the beach but away from the tourist destinations of Georgetown and the 7 Mile Beach, and we cook most of our breakfast and lunch meals. A rental car is included in the price and that's fine with us because there is a lot to see and do on Grand Cayman - including some free and inexpensive options.
On GC we tend to shore dive and snorkel and only do a few boat dives, but an all-inclusive dive resort like Cobalt Coast is another option for controlling costs. There are other diver-dedicated resorts on GC like Sunset House that offer packages. There are also small dive operations like DNS and Scuba University that may be willing to negotiate a good dive package.
The diving is even better off Cayman Brac and especially Little Cayman, and the flights to the sister islands from GC are only around $100 or $150 roundtrip. When we go to the Cayman sister islands we stay at the small, diver-dedicated, all-inclusive Cayman Brac Beach Resort or Little Cayman Beach Resort so that accommodations, food, valet diving, taxes, service charges, and transfers are included and we know the costs up-front. They also offer some packages that include alcoholic beverages. We may rent a car for just 1 day to tour the sister islands. The same company that owns these 2 resorts now owns the Cobalt Coast resort on GC.
The Caymans also offer some valuable intrinsic benefits that are not always available in other tropical locations. It is usually pretty easy to find convenient flights at a decent price to GCM. You can drink the food and water without worry. They have good mosquito abatement and very low incidence of dengue fever and chikungunya and no problem with malaria. They have good roads and health care facilities and good communication systems, the islands are safe and the locals are friendly. It is a British island so almost everyone speaks English.
But your destination choice should also depend on the things you like to do (besides diving) while on vacation. There are lots of restaurants, shopping, and night life on GC but very few of these entertainments are available on CB or LC. The seven mile beach is a world class destination, but there are some other nice beaches throughout the Cayman islands. The Caymans are also rather conservative for a tropical vacation destination. There is no gambling or casinos, no topless or nude sunbathing, and it is hard to buy a bottle of liquor on Sunday, so they may not be the best fit for everybody.
You will see healthy reefs and marine life while diving in the Caymans but not a lot of sharks. We recently returned from diving Grand Cayman and Little Cayman and we saw beautiful walls, healthy corals and sponges, clear, clean, warm (84 F) water, and lots of turtles, stingrays, tarpons, eels, octopus, groupers (including a goliath), and barracudas and lots of tropical fish and many small creatures including a seahorse. We saw some nurse sharks and a few reef sharks but not many - so if you like to see a lot of sharks this may not be the best choice for you.
Most of the boat diving is done from permanent moorings and there usually isn't a lot of current so the Caymans are not a drift diving destination either, if that is your preference. There are a few nice, artificial reef, wreck dive options available, like the Russian Frigate and the Kittiwake - but the Caymans are not a big wreck dive destination if that is what you like to do.
I also like really warm water (in the 80s) and during the winter months the Caymanian waters are usually in the mid to high 70s, so I prefer going further south to someplace like Bonaire or Curacao at that time of year. But if you like to travel during the spring, another nice thing about the Caymans is that they are not a "college vacation, spring break" destination, like parts of Florida, Mexico, or the Bahamas.
The Caymans are a great destination for a 2-island vacation. We like to spend some time enjoying the amenities and attractions of Grand Cayman and then travel to one of the sister islands for our dive vacation. Your airfare costs and travel times are almost the same when you add on some days and a second destination and you get more vacation and two different experiences.
Wherever we go we often travel during the low season which reduces costs (and crowds) but we run a higher risk of bad weather. Every destination (and season) has pros and cons, you just have to do some research and make the decision that is best for you; but keep in mind that sometimes a pricier destination may be worth the cost.