My wife and I spent a week on the T&C Explorer II in Jun 06 and then spent two weeks on the Caribbean Explorer II in December 07. I would agree with everything that's been said above.
T&C from the shore would be more boat riding than diving if you want to get far enough away from the populated areas to get to the good dive sites. Trip report at
http://www.scubaboard.com/forums/cr...-turks-caicos-explorer-ii-3-10-june-06-a.html.
We did two weeks on the Caribbean Explorer II so we went St. Kitts-Statia-Saba-St Martin and then back in reverse order the second week. The best diving by far was at Saba. The poorest diving was at Statia. Part of this was the boats penchant to be close to the drop off point (the city dock) for the mandatory underwater guide at the end of the day. This drove them to dive a place called double wreck three times out of the five dives we did there. This dive had the worst visibility of any dives we did either week and sometime a significant current (one night dive was cancelled there because of it). Also the mandatory guide once you've gotten used to going off on your own is a drag. St Kitts had two good reef dives that we very much liked but it also had two wreck dives that we did not like. We just don't get wrecks and these were more like bottom debris than most wreck dives (IMO).
If I had a choice I would do a liveaboard over either place from shore and if I had a choice of a liveaboard at both places I would definitely do T&C.
Guy