Dr. Roberto Coronel of University of the Philippines Los Banos, one of the foremost plant men in the country, insists that the Longkong in the Philippines is erroroneously labeled and that it is in fact Duku. He says it in an interview on youtube, stating that it is the perfect combination of sweet and sour.
On the other hand, dizon nursery, a nursery started by the foremost pomologist in the country, describes their Longkong as very sweet. Very few seeds, and abundantly fruiting. The likes of which fit the longkong. I only point this out to illustrate the large confusion amongst these varieties. It seems we have a discussion on this fruit once every year, and we never get a consensus.
My suggestion would be to try as many fruits you can get your hands on, and then grow that exact type. This seems to be the only way to ensure you are getting the right advantages you wants. E.g. Sweet, thick skin, no seeds.