That's a comparison that can go either way, for a number of reasons.
1.) Will the trip go? Belize has 2 Aggressors, the
Belize Aggressor III and
Belize Aggressor IV (formerly known as the
Sun Dancer II, which it was still labeled when I went on it in 2015).
The
Cayman Aggressor IV was replaced with the
Cayman Aggressor V, which was supposed to be bigger and nicer but there were complaints about it, including that it was too big for some moorings. During the pandemic, there was a thread on ScubaBoard that got into the Caymans government claiming Aggressor owed them a lot of money, one post to my understanding indicated it might be shady on the government's part (a manipulation to extort money from businesses - see
Post #73 for what I'm talking about), the CA-V was departing and some figured it wouldn't go back, but last I heard it sounded like a Cayman Aggressor (not necessarily the V) would.
I'd want current answers to the standing on that before I booked a Cayman Aggressor trip in the near term future. Let's do a little searching...
@scubadada posted a thread in Feb. 2022:
Cayman Aggressor V becomes the BVI Aggressor, IV returns to Cayman Supposed to resume July 2nd.
So Belize has 2 boats actively running; the Caymans have one, there's been a lot of drama, and plans are for it to come back early July. You plan to book for December, so maybe that's not too risky.
2.) Where will the boat go? The best diving for the Belize live-aboards is at the outer atolls; I saw one trip report where the boat spent more time around Turneffe Atoll (IIRC), and the trip sounded somewhat underwhelming.
But the glory of Caymans diving has long been held to be Little Cayman's Bloody Bay Wall and Jackson's Bight area; the Cayman Aggressor IV has to make a crossing from Grand Cayman to LC and Cayman Brac (for 2 dives on the Keith Tibbetts wreck) for the full experience - otherwise it sticks around Grand Cayman the whole week and you only dive there (note: I am not knocking GC diving).
3.) The Diving. I had 'best case scenarios' with my trip to each. I found the reef and wildlife similarly impressive, though some animals in the Caymans were more approachable (e.g.: Nassau grouper; a hawksbill sea turtle swam out to a guide). On the Caymans trip, dives included the Doc Poulson wreck, Kittiwake and Keith Tibbetts, and the Belize trip had no wrecks. Belize had the Blue Hole (which some people like, and I considered worth doing once).
4.) Alternatives. If you plan to eventually hit both destinations (recommended), consider whether you might prefer to save one for a land-based option. In discussions of the Cayman Aggressor, often someone points out a preference for land-based trips in you hit the same sites the live-aboard would. No land-based operator will hit all the sites the Cayman Aggressor IV does if it makes the crossing, but if you mainly want to dive Little Cayman, going to Little Cayman Beach Resort may make more sense. While land-based stays to dive the outer atolls of Belize are possible (e.g.: Turneffe Island Resort), I don't see people criticizing the Belize live-aboards as redundant.
Sun Dancer 2 (Belize) May 2015 -
http://www.scubaboard.com/forums/li...ving-belize-via-sun-dancer-2-may-2nd-9th.html
Dive Trip to the Blue Hole & Visit to Half Moon Caye -
http://www.scubaboard.com/forums/belize/506647-some-blue-hole-impressions.html
Cayman Aggressor IV May 2016 -
Trip Report - Cayman Aggressor IV Trip May 2016
I loved both trips.
I haven't paid full price for an Aggressor trip yet. The past several years before the pandemic, there'd been an annual 30%+ off sale covering the Caribbean region Aggressors. I think last year it was early November. That's a savings of several hundred dollars. I get that that's awfully late to book a Dec. trip, and even if you could, airfare on short notice might kill your savings, but you should know.
But what about pandemic issues, inflation and the terrible financial blow live-aboard operators sustained from prolonged shut-down related to the pandemic? Will there be a big sale this year? I don't know. I'd check for a 25% off 'dive the world' offering, but
their specials page doesn't have anything matching what you want right now.
Would you care for an alternative strategic plan. Book a land-based trip, maybe with Little Cayman Beach Resort, Cayman Brac Beach Resort, or somewhere else...like
CocoView Resort in Roatan. For your 1st live-aboard, wait and see whether one of those big sales happens later this year. If and when it does, book a live-aboard at a big savings.