RE: free formula 158 vs 155 isonic????
FF158 has earlier planing, better ability upwind (or at semi FW extreme angles both upwind and downwind, especially in marginal conditions) and will carry larger sails and fins more effectively.
iS155 has better top speed and extended top end range over FF158. IS155 is easier thru chop at speed, has a better jibe and is somewhat lighter in both weight and reactive ride.
Please let us know if you would like further specific info.
Cheers ~ Ian