A freeride board like the Futura will be faster than an iSonic in the hands of most novice and intermediate sailors. It's easier to sail and has better control in choppy conditions than the iS. Experts can reap the benefits of the true slalom boards and can potentially go faster. Overall, if you aren't a dedicated slalom racer with a lot of experience, the Futura will likely suit you better and be more fun.
Speed is a product of the sailor and his ability to control the board in fully powered, rough conditions. I think that at your level of experience, you will be faster on the futura.
Good success in slalom racing means that you don't make mistakes. Great starts, consistent planing jibes & no falls = winning. Speed is secondary.
Just my thoughts............
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.