For maximum performance (leverage) take the outboard position, but if you are sailing in really extreme overpower and rough water conditions (so almost B&J mode rather than "pure" flat water slalom or speed) then you get a bit more control (with the rear strap inboard).
As it can be a very fast board in very rough conditions, often using the inside setting initially to get the "feel" of sailing a board like this in wild conditions is a good idea. Once you have felt your way, if you really want to up the performance and go "out on the edge" just that little bit further, then outboard is the answer.
It's a fine line - in some ways lighter riders seem to tend even more to the inside position on this board in wild conditions, but the counterpoint is that the lighter rider (in ultimate performance settings) wants all the (extra) leverage they can get..
Try both - with iS76 (and conditions) it is definitely worth taking the effort to find the optimal personal setting for your size/style and water conditions.
Cheers ~ Ian