I don't have experience with the smaller Isonics, I have a new 111 and weight 80kg. I bought it for 7.5 sailing, but will use sails from 6.5 to 8.5. I sail fresh water with variable winds, so I like something that won't sink if the wind drops to 5 knots.
At 70kg, I think you are fine with the 86 if you have consistent winds in the 15 to 20 knot range. If you have marginal conditions, the 96 will keep you afloat in the big holes, but you will get more range with the 86.
Since you have been on a 90 L board and a 6.5 sail, you need to decide if you are more likely to go down a sail size (iS86) or up a sail size (iS96).
Hopefully, some other sailors with more experience with the isonics can offer some advice.