RE: iSonic or Carve as Lightwind Board
Just a further thought from an average sailor about maiximizing a board quiver.
Consider the largest sail you use (happily) with your 104l. For the next board size up (be it carve or isonic) go for a board size that will take that same sail as the smallest sail (that the board performs well with - not nessararily the smallest the spec say).
It's easier to change a board than a sail if the wind changes. I got a starboard this year and am impressed with it's range.