My take on this is that you will not plane in 7 or 8 knots with either combination -
iS 150 with a 9.7
LWR with a 9.7
In 9 - 10 knots and good pumping, you may plane on both boards. The only way to plane in under 9 knots is with a formula board and 11-12 meter sail for someone weighing 88kg.
I weight 78 kg and have an 11.0 on a formula 160. A steady 9-10 knots and I can get going without too much pumping. Under 9 knots, it's a struggle, any holes and I come off plane quickly.
All of the above assumes you sail in fresh water. Salt makes it a little easier.
In the range of 6-10 knots, there are lots of variables that impact planing or not planing - Actual wind speed; water state / chop; fin; board; sail size; rigging / tuning; mast; boom; pumping skill; experience. The point is - it's not an exact science and all we can do is make estimates (guesses) based upon available information.
Hope this helps.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.