I've sailed a few versions of the 101 but not the 111. I'm 70kg. It is hard to say without comparing, but my thoughts after having sailed the 101 is that I would not gain much by going for the 111. 7.5 feels spot on on the 101 and you can use bigger too. At first the iSonics (and the 101) can feel kind of small and not good for early planing, but when you learn how to sail it is planes very early and then has a fantastic ability to glide through lulls. In my opinion, the trick it to very early get pressure on the fin. SO you can no pump to furiously at first. A few careful pumps and then when you feel some fin drive which helps lift the front part of the board out of the water. Then you can pump a bit harder "against the fin" to aid acceleration.
I don't compete, just "freeride" my slalom board. If I would compete a lot in light wind. I would probably go for 111 or even bigger. But for freeriding and also considering the total range of the board, I do think the 101 is the best size at my weight.
If you either prefer a more passive style or a more hard pumping sluggish style, the bigger 111 might get you going earlier. But if you develop the feel for the board, the 101 should plane about as year with a 7.5 and then offer a more controlled ride as soon as you are powered up. At least that is my guess after using the 101 (2007 and 2008).