I have isonic 105 dram, I just use one fin under the board:
Tectonics Goldwing 36
It works from very low wind to high winds, fin is very controllable and easy on full speed. Never had real problems, see my gps results from yesterday:
Another good option is the Select SL7, I tryed it and they rock also! The big advantage of the select fin is crosswind speed... they are f*cking fast. Upwind angle is also verrrry good.
For the 115 I would go:
-Tectonics goldwing 38
-Select SL7 37
-Deb SL2 38