My thoughts, which may be of some help.
I have an iS 111 and a similar volume HiFly bump and jump board. I can't speak directly to the iS or Futura 101's, but I feel confident that the Futura will be the more enjoyable board for most freeride sailors. Only those committed to racing slalom or that like to master challenges will love the iS.
Because the iS has a wider tail, it is a little more difficult to jibe, but it will plane quicker than the Futura. There are a lot of board design issues that determine how easy a board jibes, but that could be a lengthy discussion (tail width, tail volume, soft or hard rails, fin, flat or V bottom, cutouts and rocker).
The iS board rides rougher than the Futura in tight chop. Open water chop is spread out enough that the iS does fine. When maxed out, the iS can be a very scary ride in rough water.
The outboard straps on the iS allow a lot of leverage on a larger fin, not too unlike a mini Formula board. It goes upwind pretty well. The iS doesn't jump or chop hop well because of the outboard straps.
The iS is faster in protected flat water and small chop, but in rough water, sailor skill will determine which is faster. Intermediate or even advanced sailors will probably find the Futura faster in rough water, because it is more forgiving, smoother and easier to sail.
Hope this helps. I too would like to see what others have to say about the board comparison.
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.