If money is no object the best location I have seen for learning is Long Bay Turks & Caicos. Lessons and lodging are expensive, but there are many miles of waist deep or shallower turquoise waters with a soft sandy bottom. It is where I first took lessons, and I have just returned from another 5 days there. Winds this time of year are spotty but improving, but I would argue are just fine for learning, since you need to spend time with a kite before a board, and light wind hones kite technique more quickly than heavy wind. I will give a shout to OtherSide Boardsports, with whom I have not kited, but with whom I have had very positive experiences in the past - and Islamorada is great. I have not been to Real Watersports, but if I were starting out, I would think that would be the place where one would get the most consistently high level instruction, and the conditions are great (shallow flat water, good wind, great facility).
After the several lessons on TCI when I was starting out, I headed to Cabarete a month later. I would not recommend Cabarete for beginner lessons - it is chaos, with hundreds (and sometimes 1000+) kites on the water in a limited area, with deep water, a breaking reef, and sloppy chop. I would also say that my experience with lessons there was awful - I was basically handed a kite and sent out on the water, and told to bend my knees more. I twice had other novices wrap their lines around mine while I was in the water with kite at 12, with one of those experiences leading to a long swim attached to a flagged out mess of lines and flapping kite. Cabarete is cheap (though credit card fraud made it more expensive for me) and very, very windy, and there are lots of kiters there from all over the world with whom to share experiences, but it is unforgiving for the inexperienced. Having said that, I am headed there again in three weeks - the choice of friends with whom i am traveling (this time I take lists of cash, plastic goes unused).