I was in the same position as you, I had a formula 160 and a HiFly 105 move and wanted to fill in the gap with something in between. I weigh 79 kg. I do race, both formula and slalom (slalom occasionally). I am an amateur, age 64, but I have been racing for 25 years.
I picked the '08 iS 111, but seriously considered the Futura 111. The occasional slalom race pushed me to the iS. I haven't sailed a Futura, but from what I have read, it is more user friendly, a smoother ride and just about as fast as the iS.
The iS is fast, but it's a pretty rough ride when powered with my 6.6. A little smoother with my 7.6 and comfortable with my 8.4 if there is enough power. Slogging with the 8.4 is tough and a little difficult. The board wants to point up and it's hard to keep it off the wind or on a beam reach. Since there is little floatation in the nose, it's hard to push it off the wind in 5-10 knots. Not a sinker, but not a lot of floatation either. Uphauling is pretty easy with my 6.6 and 7.6 sails, but a little more difficult with my 8.4.
The iS will plane a bit sooner because of the wider tail, but probably not much different. It has outboard straps similar to the formula and it's a little difficult to get onto the back straps since they are so close to the rail.
I have gone over 30 knots (board speed on my gps) on both the iS and the HiFly. The HiFly is much more comfortable and less scary, but the iS is a little faster (1 knot).
All my sails from 6.6 up are Maui Sails TR race sails.
Formula 160 - 11.0, 9.2, 8.4 sails
iS 111 - 8.4, 7.6, 6.6 sails
HiFly 105 - 5.7, 5.0, 4.5 sails
In my opinion, if you like the challenge of mastering a full on slalom board - go for the iS. If you are looking for something more traditional and more user friendly, go for the Futura.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.