Only you can answer your question with the info provided. Obviously the width is the critical factor.
iSonic 96 = W59
iSonic 101 = W64
The 101 will be better for 15 to 18 knots. The 96 will be better for 19 to 25 knots.
If your primary use is the lower end of your range, then go for the 101.
If your primary use is the higher end of your range over 17/18 knots then go for the 96.
Personally, I would buy the 101 then get a cheap waveboard for high winds. Don't forget that 1 knot is a LOT of wind. When you are talking about 20 knots, I think you would have much more fun on a waveboard unless you have very clean slalom water conditions.
But I am not sure if you are talking about TRUE wind strengths or just the FALSE wind strengths of the tossas on the beach.