I have been to both several times, but not to the new Mahogany Bay at Roatan.
Grand Turk--Paul is right, the cruise center is where you will want to stay unless you want to doa horseback riding or dune buggy tour. We rented a golf cart (and got lost cuz of a crappy map) and it isn't that pretty and we never did find the historic old town after searching for it for over an hour. If you do go there the water tours are really good. There is good diving and snorkeling and Gibbs Cay just of the East side of the island is where the stringrays are and the beach/water is phenomenal white/soft sand!!!!
GrandTurk also have a massive Margaritaville, with prices to go along. The pool is fabulous and the flowrider is near the pool where you can watch the people attempt to surf/boogieboard....
Roatan--It is very pretty and scenic on Roatan. I have done two tours that covered the island pretty much and I enjoyed myself. I loved the bird and butterfly santuary. I was last there in January 09 before the new Mahogany Bay was where Carnival ships docked.
Both routes have advantages and disavantages, but I think if you look at the other ports you can decide. I loved Belize and if Half Moon Cay is part of the route then I would not hesitate to tell you to RUN to the ship that goes to Half Moon Cay as it is an amazing day at the beach stop. It is a tender stop so getting off and back on has to be timed right (get off in the first two or three tenders or book a cabana, and go back early enough to avoid a 30-45 minute wait for a tender and then security once onboard.
Good luck and have fun!
2007 Victory & Triumph
2008 Conquest & Liberty
2009 Valor, Freedom, and Splendor
2010 Glory and Liberty
I gotta go where it's warm.....
Over 40 victim of fate.....