The Sonic 85 is not really more relaxed than the Isonic 87. But I might follow what you have been told, that it should feel a bit more "relaxed". The 85 don't have the small cut-outs, and the board trim will be a touch more "steady". The small cut-outs on the Isonic 87 allow the board to ride a bit more "free" on the tail. This can might bee seen as a little more "reactive" sailing, but I would not say it’s a harder ride than the 85. It al comes down to the trim (track position, fin, boom height etc).
Again both boards are very good.