I agree with Floyd. Sailor weight does shift the sail range of a board a bit. Not a huge bit but a bit. Since the iS101 is already quoted to handle an 8.5 so I think you will definitely be on the safe side with 70kg and an 8.3.
I'm 70kg myself and used to have the iS101. My biggest sail was a 7.5 but that didn't feel anywhere near max, rather quite sweet spot, ie the board felt effective and easy to plane on at the lower spectrum of the range of the sail and controllable and smooth in the upper range.
If top performance with the 8.3 in a racing situation is important, my guess is that the 111 will be a tad better. But overall I feel the 101 is a much better match to your weight.