Hi again...this is getting almost academical but I kind of like it.
You concetrate yourself on a same sail size. But to me it is more reasonable to take the wind and fix it.
And now speeking from my own experience, I would pick the board where I would get like 15% below the max recommended sail to be overpowered...as a 100kg guy (appropriate combo)...accordingly to your weight you get a smaller sail and be the same way overpowered and now to the board...I would rather go for a smaller volume on to get better speed once planning (radical jibing, easy chop handling and so on...)
From your example...
let me say:
115 and 6.1 and I would be slightly overpowered (ideal condition to my experience)...
if you want to be powered in the same way you will be good on with smth like 5.6 at yours 70kg.
As far as the wind is enough to get me planning I can handle bigger sails easier than you...because once plannnig you let all your weight to be caried by the sail and you can handle more power....see the speedsurfers...they are not exactly lightweights.
What I am trying to say...if you have to go above the sail range (what ever your weight is) the ride will not make as much fun...and in fact you can say that as a lighter surfer you can handle a bigger sails tham max reccomended.
But this only is right when the havier one does not have the techique to get himself plannig (this would be probably my case...but I have enough dimensionalysed boards to get comfortable with a bigger sail size).