If you want to be closer to the Formula performances (I suppose that you have a race 9mē sail) is the iSonic 150 or 133. The 144 is more recommand for heavier personnes. If you want to race in Slalom 42 event the iSonic 133 will be the choice at contrary the iSonic 150.
After you will have the Futura 155 or 144 who are more easier but less planning ability.
All the best