This topic comes up quite a bit. Not just between Futura & Isonic, but between a slalom or a freeride board. Telling another person what board "they" want never made sense to me so here is a list that might help you.
Why would you choose an Isonic?
1. I’m in a hurry to get from point A to point B. I value VMG over max GPS.
2. I like to use powerful sails and powerful fins.
3. I like to sail overpowered with a big sail on a small board.
4. I can do a very good shortboard tack.
5. I don’t throw any freestyle moves in between point A and B. See #1.
6. I like to be physical and pump onto a plane.
7. I sail for short stretches and take plenty of breaks. Comfort is not important.
8. I want to win races.
Why would you choose a Futura?
1. I’m in no hurry to get from point A to Point B but I want good GPS speed.
2. I like to use fast, swept fins and fast sails with few or no cams.
3. I like to sail efficiently with a small sail on a big board.
4. I may not be able to do a shortboard tack.
5. I like to throw in a few upwind & downwind freestyle tricks in between point A and B.
6. I’m not a big fan of pumping and would rather coast up to a plane.
7. I’m on the water for a very long period of time with few breaks. Comfort is important.
8. I’d like to be competitive in races.