RE: S-type 104 vs 115
I've sailed both boards extensively in the last year. Even though the 115 doesn't have much more volume it will carry a bigger sail better. The 104 at my weight, 84 kg, starts to bog down with sails much bigger than 6.6 while the 115 carries an extra 0.5 meter quite well. At your lighter weight you can make somewhat bigger sails work, but the 8.2 will be big even on the 115. Regarding control in 12-20 the 115 won't be too big. Considering what sails you want to use, I'm inclined to recommend the 126. One of my sailing partners who is close to your weight and height has one that he routinely uses with a 6.6 and 7.6 Neutron and he doesn't find it hard to control on very choppy water in the wind ranges you mentioned.