When you're riding in those conditions, what is your primary ride style ?
Are you really into racing your friends ? Or more like moves and tricks ?
While the numerical difference in wind strength over 9 to 15/16 kts is minimal, the variance in effective /available wind power (and the effect on subsequent board size) is significant.
Pushing below 10 kts will test the planing threshold of most(nearly all) short boards.
Power from the wind effectively halves between 10kts and 8kts of windspeed (theorists, pls take it easy, generalisation for discussion..) so every kt you push below 10 really hurts. Obviously light rider weight helps reduce the reliance on extra volume in this case, but the most efficient board (for maximising early planing+ bottom end range vs size/volume) will be the race boards like iSonic.
New iSonics are certainly not too difficult or technical (especially in the light wind range) for consideration as general "hi efficency" freeriding.(might have to stop short of suggesting freemove potential etc..)
However, if you have a greater emphasis or interest in "moves" type riding, then yes, the Futura is probably the better choice - it concedes a little bottom end/early planing (c/w iSonic for a given size/volume) but on the upside, obviously an easier and more playful fast freeride/freemove once powered up.
In either case, the 122 sizing is about right. Obviously at 9 kts wind, you want/need all the board you can get for solid planing(could realistically be on 133 for that), but by a true 15-16kts, you'll be looking for a fair bit less size (think 101 in both families), as a 65kg rider is going to be bounced around by a bigger board earlier than heavier riders.
The key will be to understand where (in the 9-16 kt range) you will spend the high/er % of your sailing time (and maybe further still, your ENJOYABLE sailing time), then tweek the sizing accordingly. Taking a Futura over an iSonic in this range, you could argue to take a size biiger in Futura than iSonic, but it's probably more like "half a size" bigger is more correct. I'm not sure you would look real happy on iS111 in 9 kts ! - but to ensure solid planing on a Futura in 9 kts, I wouldn't rule out FU133 either.
In any case, for sure the use of 2 or 3 fins to help cover the range variance will not only be helpful, but probably the best (and most cost efective) advice we can add.
Cheers ~ Ian