Opinions will vary and the bottom line is that you could buy either one and do just fine. If it is the iS, it will be more of a challenge to master. The Futura will be easier and more fun in my opinion. Top speeds will be very close so I wouldn't let speed be the deciding factor.
Your personality and commitment should be the guiding force. If you are very agressive and dedicated and may race slalom, then the iS may be the board. If you just like to have fun and blast back and forth, then the Futura is the board.,
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.