For me at 85 kgs, the isonic 125 can pretty well be sailed with 9.8, but the difference in planing abilities compared with my RS Slalom 8.4 is small- if even existing. So I prefer the 8.4 even in the lightest winds.
I did own the s-type 104 and wasn't able to manage bigger than 7.2 on this board, 7.8 was to big and 9.0 ... ough. But smaller than a 5.4 didn't work for me, the board got bumpy. In fact, my futura 101 handles bigger sails (RS4 7.8 on F101 is great) AND smaller sails better than the s-type 104.
So apart from the weight stuff, preferences, surfspot and personal style is a big issue.