I've had both F101 ad iS101. The F101 is a fast and effective board and super easy to get planing, but if you have the technique (getting pressure on the fin very early) the iS gets going earlier and also accelerates faster. In high wind, at last if its choppy, the F101 is fur sure MUCH easier to control. At least for me that does not have big slalom balls. The F101 handles chop in a kind of soft and stable way regardless of how you sail it. On the iS101 you have to keep power on and really drive the board or it will start to bounce and become unstable. As long as you do this, the iS101 is also very nice in chop though. But for me, at 70kg, fully lit 5.5 is just more than I can (or dare to) comfortably handle on the iS101.
So the short answer: It depends - it's a technique issue at both ends.
If you're heavier than me (or better at holding on...) I think you will have no problems with the iS101. If you're light like me, I would think about having something like the iS86 as the smallest slalom board and save the 7.8 for the iS125.