Of course the formula board. While the iSonic will do well, it won't keep up with the formula. The 70 cm fin and 100 cm wide board makes the difference. For free sailing, the comparison is of little value, only while racing would the comparison of the two be of importance.
Choppy or flat water won't change the results.
If the iSonic was faster on a formula course, the top sailors would be racing them.