Unless you're obsessive about riding the smallest board possible in the conditions (can be a valid consideration in super choppy locations) then the iS125 will be the sweeter of the two for 7.5 thru 9.8m.
Realistically, even on the iS125 (122) the 9,8 is going to be bigger than ideal, and something like a 9.0 sweeter as a light wind sail on that board. (9.8 on iS111 /115 won't work well, at all. No test ride in the boonies required for that one!
Let us know if you need more info.
Cheers ~ Ian