I weight 70 and have both Futura 101 and iS101. I'am really a wave sailor and only got into slalom and freeride boards recently and don't really have the best technique. More exactly, I think I can get the stuff going very early and so on, but I probably lack the power attitude to handle slalom stuff in chop when super, super powered up.
That said, the iSonic is the most effective board in light stuff when you know how to work it. You can definitely feel that this board has been designed primarily with efficiency in mind. You can get away with smaller fins and still be reasonably effective upwind too. The Futura feels very effective too, but find it need a bit more power to reach the level where this efficiencey comes onto play. On the other hand, the Futura requires less technique to get planing - you don't need to drive it of the fin so much and can pretty much just stand there and the board will get going anyway.
In the mid wind spectrum, both boards are very nice and easy. Probably the iS has an edge in pure speed and in upwind, but when just sailing around that not something you really feel.
When the hammer is put down, the iS for me starts to get a handful. It just feels like a big board and currage to hold on becomes the main factor in choppy donwwind runs. No doubt, a good slalom racer will be able to sail on no worries far longer, but as I wrote, I'm not that sailor yet. Upwind and half wind is still rather smooth also in real power mode though. In the same conditions, the Futura just feels as composed as ever. I haven't done any comparative GPS clocking in these conditions to see if I really sail the Futura faster, but I imagine that the more powered up I get, the better the Futura would go.
Jibes and stuff is very smooth on both boards actually, and both respond very well when you put a lot of power into the turn (here the wave sailing background is a plus). Even the iS101 cuts through chop nicely. But of course, the Futura is still an easier jiber and the less good you are in keeping a board on its rail through a turn, the bigger the difference between F and iS jibing will become.
So, which one to choose. Well, I don't know. No doubt, the iS will be a more effective board. If effectiveness is what makes your day, then it will be more fun too. But if you like powered up freeriding and power through big chop, throw in a jump here and there and some jibes for fun, then you're in Futura territory. So what KVDA is hinting at above certainly holds: the Futura is a very fast freerider probably capable of beating a slalom design in some conditions and for some riders. But the iS is a dedicated slalom board and for slalom like sailing, thats hard to beat.