Tension doesn't effect stiffness at all... so long as you don't get to the point where spokes go slack. Higher tension means the wheel will take a larger load before it gets to that point.
Bracing angle is the most important factor for stiffness... along with rim stiffness, with spoke stiffness farther down the list.
J or straight pull should make no difference if the wheel is built properly. I'd probably go with the J and cross lace both sides just to improve the longevity of that flange. You could bump up the tension a little but I wouldn't go past 130kg.
What spokes and rims?
Okay, that makes sense, I'm 65kg so hopefully I won't need super high spoke tensions so it doesn't become flexy under hard braking. The hub I'm looking at also says to radial spoke the J bend one on the non disc side. And tangential lace the disc side. So same lacing pattern as the straight pull. Would this lacing make a weak wheel compared to if I got a tune and did 3 cross on both sides? Weight difference is around 40 grams.
Rim's are the light-bicycle wide 29er rims, 24-25ID and 30mmOD then spokes will be Sapim cx-ray. I have red alloy nipples but not sure if I'll use them or brass since it's a carbon rim. So hard to give up the red colour and 45 grams at the outer point of the wheels.