With a solid 12-18 kts/7.9m/87kg active style, you should be very sweet with iS122/125. In fact, ideal, in your conditions and to quiver with ST104.
Many guys who prefer the easy speed of the ST in higher winds/rougher conditions still prefer the extra efficency of the iS as a bigger "blasting" board in lighter (and less choppy) conditions than their mid-hi wind ST blaster.. It's very logical and very practical.
While the iS133/135 offers some early advantage over 122/125 to actually get planing in lighter conditions, if the conditions are choppy (as you mentioned) then -once planing and up to speed- most will prefer the "smaller" ride over the chop (in 12-18 kts) from the 122/125 c/w the 133/135.
Have fun !
Cheers ~ Ian