I understand, and the dilemma you face wasn't much different than when I decided to buy my '08 iS 111. I debated for some time if it should be the iS or Futura since I rarely race slalom (mostly formula).
I finally went with the iSonic for three reasons. One, it would be a bit more challenging to master (I saw this as a good thing). Two, the 111's sail range fit perfectly into my board and sail line up. Three, the iS would plane up quicker and stay on plane longer than the Futura. I sail in puffy conditions with lots of holes.
The down side of the iS is the rough ride, and it's not good for chop hopping (at least for me). Also, the back outboard straps are hard to get into. However, it is an exciting ride and if the wind is steady, no one passes me.
I also agree with mark, the right fin(s) makes a difference, but you probably have already figured this out.
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.