All good location choices for your trip, and I don't think the weather that time of year would affect my choice, as it's getting into a very nice time of year to dive anyplace in the Caribbean. But they're such different places/diving/trips, it really depends what you would like best and there's not enough info to guess on that. I infer you're looking for not too challenging yet, which really most Caribbean diving isn't. Do you have a strong preference for boat vs. shore, or how many dives a day you think you want to do? If you're land based do you plan to rent a car?
I think Grand Cayman is different enough (above and bellow) from Little Cayman that you wouldn't feel you are repeating a trip. I'm not a huge fan of Sunset House. It's a great dive hangout atmosphere-wise, but we prefer nicer rooms and grounds if we can get them, and there's other options on Cayman we can get that and better diving. The shore diving is pretty convenient, but it's kind of beat. And they dive the West wall most of the time because of their location, which is my least favorite area to dive on Cayman. You may decide SH suits you, but I much prefer the mentioned Compass Point (no shore diving there) or some other places on the island. We haven't stayed at Cobalt Coast since it changed ownership and became part of the group with the sister islands resorts LCBR and BRBR, but that may be worth looking into if you haven't. They (CC) also have shore diving on site, though it's a longish swim and gets blown out much more often than someplace like Sunset House because of the location. But that location on the NE corner of the island allows you to access better diving, and there are a couple more sheltered places to go shore diving nearby if you want. Divetech is a very good op, though not "valet" like some others. They have shore diving at Lighthouse Point Condos which are on the pricey side, but also have a location at the Holiday Inn where the prices are reasonable and I think at least some of their boats leave directly from there now. There are tons of options on Grand Cayman, depends what kind of place and routine you want. Many people rent condos and get picked up for diving by one of the many good dive ops that do that. Having a kitchen has it's benefits and can save you a good amount of time and money, even if the condo costs more than a hotel room it may all come out in the wash anyway.
Bonaire is a good place to build experience on your own. Easy diving, easy to navigate and currents unlikely. Many people will imply Bonaire is only about shore diving and that's all they do there, or it's all you should do there. But boat diving is certainly available if you want it, and the boat diving is cheaper there than most places. Personally we always mostly did boat diving there, as we're not fans of hauling our gear all over the place. Different strokes and all that. We're actually also fans of Curacao over Bonaire, for the better flights, restaurants, and variety in the diving. I enjoy underwater structure as much as the life, and Bonaire is pretty monotonous in the UW topography department.
Belize is a great place to do a liveaboard as you get to spend all your time at some of the best diving in Belize, which is time consuming and expensive to get to if you're land based. I prefer the cabins and other inside spaces on the Aggressor IV over III, but both are good boats. My only complaint is the day they do the Blue Hole, because IIRC they only do 3 dives that day including the Blue Hole, and have a picnic lunch on Half Moon Caye. Thing is, I have no interest in wasting a dive on the Blue Hole, nor do I like picnic lunches on hot sandy buggy little islands, so I feel like it's wasting a day of my trip, and it deters me from doing it again. Of course if you want to do those things, then it's great. If you don't, it's still worth at least one trip. (I've done it 3 times, but one was a special itinerary, and another was to go with freinds who hadn't been there.) So if you're interested in trying a liveaboard and can get spots when you want to go, overall well worth doing.
