Formula has more tail width, wider stance and harder rail, all of which improve the performance at lighter winds, early planing and upwind in combinattion with bigger FW fins and sails.
The iS155 has a narrower tail, slightly softer rails and an increase in control/comfort levels in choppy, more powered (by ~150Lt) conditions and an easier, more natural (for 150Lts ) jibe..
Please see also the discussion on the FW vs 155 at
and let us know if you have any further questions you would like resolved.
Cheers ~ Ian