Someone from the "team" will probably have a more precise answer, but I was in a similar situation a while back. I have an iS 111 and a HiFly Move 105. Volumes just about the same, but the iS for me is best with a 6.6 or 7.6 sails. I have used a 5.7 and 8.4 but don't care that much for how the board handles with them. I weigh 78 kg.
If the wind is over 20 knots, I go for the Hi Fly with a 5.7, 5.0 and sometimes a 4.5. It jumps well, rides smooth in chop, jibes well and is pretty fast. I suspect that the Futura 101 is much like the HiFly but a little wider.
In your case, the difference between the the iS 101 and Futura 101 is that the Futura will offer a smoother ride and it will be much better for jumping and will be a bit better with smaller sails.
IF you don't have a board smaller than the iS 101, then the Futura 93 may be better option if you have steady winds over 18 knots. I picked the HiFly105 because it is somewhat easy to uphaul. I also have an 85 liter board when it really blows.
Hope this helps