Do a quick search and you will find this topic debated many times. The short answer is buy which boat you like the best. Of the main 3 manufacturers, no boat has any clear advantage over the other and only having a marginal improvement when the conditions are suited to that boat.

If you find you are in an area that has a large Nacra fleet, go for the Nacra. Same goes for Hobie and Capricorn etc. This means you should have the support of the guys who are sailing your exact boat and should help in learning how to sail the thing.

Tiger Mike