I would choose Roatan over Grand Turk. We have been to both places twice. If you do not snorkel or scuba then Roatan has more to offer. And even if you do snorkel then the reef at Tabayan Beach in Roatan is outstanding. The reef is right at the beach and beautiful. in Grand Turk you have to get to any reef by boat and while nice they don't compare.
But you really have to look at the other stops as well. And if there are more than one that you don't like and don't really like to stay on the ship then go the other route. Half Moon Cay is a beautiful beach but if you don't want to do water things then there is not much else to do. And yes you have to time your return to the ship or you will have a long wait.
Caribbean Trip Reports
Star Princess 2005, Sun Princess 2005
Caribbean Princess 2006, MSC Lirica 2006 , NCL Pearl 2007, Majesty of the Seas 2008, Carnival Destiny 2008, MSC Lirica 2009, Carnival Valor 2009,Carnival Legend 2010, Carnival Liberty 2010,Carnival Fantsay 2011,Carnival Valor Feb 2012, MSC Poesia Dec 2012, MSC Poesia April 2013, Carnival Legend Nov 2013, MSC DIVINA Jan 2014, MSC Divina Oct 2014