Hi Mikee,
I am also around 64kg sailing in onshore conditions in the Mediterranean.
I have been riding the Grip 76 2018 for 10 months, paired with 3.8 and 4.5 sails, from underpowered 16knts to overpowered 30+knts and from flat seas to mast high waves.
This board is the best I have ever ridden, predictable, friendly, confidence builder, fast yet loose, with nice fins, great footstraps, attention to detail, cool looks, overall a quality and well-researched product.
Having said that, its sweet spot, for my weight and conditions, is around 20knts with a 3.8. In more that 25knts it is still controllable but it gets uncomfortable due to its width and the heavy chop.
Therefore, I support Craig’s suggestion that if you regularly sail in 20-35knts the 68ltrs would be a better option. If not, then go for the 76ltrs.
Regards,
Christos