Caribbean Cruise

We've never done a Caribbean cruise but am now giving some serious thought to doing one over Christmas and the New Year. The obvious choice is, of course, the straightforward QM2 one but on looking at an online website agent they are offering a number of variations on a theme; ie to and from New York on the QM2 with the filling a different ship to the Caribbean. All offer some time in New York which always appeals.

So my question is: what criteria do you use to chose which Islands to visit. Or perhaps you would look at the ships on offer and make your choice that way? We are looking at Royal Caribbean, NCL, HAL and Princess!!!!

They don't make it easy. :D