Turks & Caicos diving tends to be rather deep and I thought the reef looked better at Little Cayman and the outer atolls of Belize.
Turks & Caicos Aggressor II Report Apr. 2018 -
Trip Report - Turks & Caicos Aggressor II 4/21-4/28 Trip Report
Turks & Caicos Research Notes -
Turks & Caicos Research Notes
I really enjoyed my Cayman liveaboard trip, but as Scubadada pointed out, there's no guarantee you'll make it to Little Cayman, which is said to have the best diving. Liveaboards are often sought when they offer access (or better access) to really good diving than land-based options. Since Little Cayman has reputable land-based alternatives, and you don't take such a risk of missing it entirely your week, going with, oh, say, Little Cayman Beach Resort might be a safer option.
Cayman Aggressor IV May 2016 -
Trip Report - Cayman Aggressor IV Trip May 2016
The
Roatan Aggressor IV gets good reviews, and at times gets to places a single land-based resort probably won't, but there has been discussion about how much such benefit you're likely to get. If you want to dive some of Roatan's best heavily, you've got the option for CocoView Resort, so you may want to do some comparison reading.
CocoView Trip Report May 2021 -
Trip Report - CocoView Resort May 8-15, 2021
My Research Notes from planning a Roatan Trip -
My Research Notes from Planning Roatan Trip
Which brings me back to Belize. People wanting land-based often go for Ambergris Caye; people wanting mainland adventure sometimes get suggestions to consider Splash Diver Center in Placencia. The best diving is alleged to be in the outer atolls region, which gives you some options - 2 Aggressors (I linked you my trip report in an earlier post), Turneffe Island Resort, Huracan Diving Lodge (no air conditioning, IIRC?), maybe others. But I don't see threads arguing that land-based here is a better deal.
So I recommend Belize. Varied topography (but had no wrecks), benign conditions, excellent Caribbean diving, and yet...I will warn you there are no guarantees you'll get quite what I did.
@Trailboss123 has produced excellent trip reports, and his trip didn't quite match what I saw -
Belize Aggressor III Trip Report – April 21-28. They spent more time (and dives) at Turneffe Reef and I don't know why.
I'd like to know how much they're hitting Turneffe Reef these days?