All of the boards in this seslection won't really work effectively in Bft3 (below 11 kts) especially if the conditions are choppy (which will make the board/s harder to plane up). Considering though that the condition might be less choppy for the new boards range Bft 4- 5 (11-20kts) than in Bft5+ (ST104) range, then the added efficency of the iSonic is probably a very attractive light-medium slalom conditions option.
To some extent the choice also depends on your ride style : if you really want a board to throw around and do some moves, then the ST is the better choice, but if you're more drag race/slalom in those conditions, then the extra bottom end of the iS (size for size) is also a good bonus - and iS doesn't handle to bad thru chop, especially moderate chop. The ST is an "easier" (less concentration) ride thru chop, but iS is very capable - and once you have the technique (not difficult) it's also pretty autopilot.
Considering the range of the ST104, the logical iS quiver choice would be iS122/125 - and if most of your Bft4-5 is more like Bft4 7.9m, then that would be the ideal size, with practical but not excessive range overlap to ST104. To get similar bottom end from the ST, you probably want ST137 more than ST126.
iS111 /115 is also a very good 7.9/6.7/87kg choice (looked at in isolation, as a single board, it would be the choice) but with the range of ST104 very close - and an excessive overlap at the top end (into ST104) and less range at Bft3-4 than iS122/125, the iS122/125 is a better quiver choice with ST104.
( Sidebar : In case you're really trying to push into Bft3 with this board , and plan more often in Bft4 (more than 5) with this new board, and especially if you like an "easy" planing style (rather than active style) , then you could even consider the iS133/135. That would still be practical with ST104 (minimal but OK overlap) and would give the best opportunity at the light end of range.)
Please let us know if you need more info etc
Cheers ~ Ian