I have the iS 111 and weigh 79kg. Best sails are 6.6, 7.6 & 8.3, with the 7.6 my favorite. It iS is wider in the tail than the Futura, which helps it plane quicker, but it is a rougher ride in chop. the iS foot straps are wide on the rails and harder to get into than the Futura. I haven't sailed a Futura, but I am sure that the iS is a more technical ride.
A year ago, I had to make the same decision as you and selected the iS because if the wider sail range, plus I wanted to master the more technical board. I have a 105 liter bump and jump board when the wind allows me to get on my 5.7 sail.