The iSonic is a race "slalom" board for beam and broad reaching on a slalom course. It will jibe better and be a little faster than the formula on reaches.
The Formula board is made for upwind and downwind racing, pointing as high as possible and running as deep as possible with very large sails.
The tail width of the formula is 17cm wider than the iSonic 150, which is the most distinctive difference between the two boards and allows them to do each of their things best.
Both boards can handle higher winds than the Go, but both are somewhat technical to sail. Both will be a handful in 20 knots and a 9m sail until you have quite a bit of practice under your belt.
The Futura 155's tail is 6 cm narrower than the iSonic 150 and is less technical to sail. A better choice for the free sailor than the iSonic, in my opinion.