I would go with the 5 nighter. I hated Costa Maya and have been to the Bahamas so many times that I don't usually even get off of the ship there anymore. I do love the Grand Caymans, though. The Liberty is a great ship but the port of call are definately not the best. The Turks & Caicos are very pretty. We usually do a 5 nighter around Christmastime and love them.
We will be on the 5 night Elation to the Bahama's next month with the ports you asked about. I can't really offer any help now, but the end of November I can tell you more about it. They all sounded good to me. Better than Mexico. I've heard good reviews on here for these ports also. It took me months to decide which cruise we were going on and which ports I wanted to visit. You will go nuts trying to figure it out.
Carnival legend November 2009
Carnival Glory October 2007
Carnival Elation November 2006
I'm sure you would enjoy both. It's all about being on a cruise. Half Moon Cay is a private island which was fabulous and Grand Turk was great. We went on the Fantasy 5 night which had the same itinerary as Elation and had a great time. I would lean towards the Elation.
We just took the Fantasy cruise Sept. 25-30. I wrote a short review about it on this site. We had a great time. The Elation is taking the place of the Fantasy, which is moving to New Orleans. I think you'll have a wonderful time on this ship and visiting these ports.