In Deboichet, most iSonic riders prefer the overall efficiency of the classic SL2.
SL 3 has been working well in choppy downwind slalom for extra control.
For your size/sails/conditions, typical fin sizing would be :
iS111 / RS 6.2 SL2 34cm
iS111 / RS 7.2 SL2 36/38cm
iS111 / RS 8.4 SL2 42cm
iS133 / RS 8.4 SL2 44/46cm
iS133 / Vap 10.0 SL2 50-52cm
Sizing here assumes slight emphasis to speed/downwind slalom, and in all cases can be +/- 1-2cm depending on many variables -or simply building a most fin (cost) effective quiver.
Obviously if you're really well powered on the 10.0 you would be able to go smaller on that fin, but in most cases simply getting/staying planing is why a sail this big would be used on iS133, in which case the optimum fin needs to be bigger to offset the large sail.
Cheers ~ Ian