You'll have to check yourself to see which airfare is cheaper for when you want to go. Airfares are ever changing and its impossible for anyone to tell you which is cheaper.
However, the cost of travel (in both directions) from FTL to Miami is much more expensive than from the Miami airport to the port (about $25 per cab) We stayed at a hotel near Miami airport which offered a free shuttle to the hotel from the airport (which saved one taxi fare) then we took a taxi to the port ($25)
Keep all that in mind when you are comparing prices.
Be careful with the tour companies right off the ship. Especially in Cozumel. The bigger hotels will be inside the port area and try to sell you an "all inclusive" beach day or something. You are better off getting out of the port area to where the taxis are and negotiate with the taxi/guides outside for better prices (yes you can save a lot of money)
what ports are you going to? Belize/Cozumel/Grand Cayman?
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013