My educated guess (after riding 08 75 wood and various constructions of the E70+80) is that in underpowered conditions and overpowered conditions you'll notice a difference. In light wind, the wood carbon just feel more active and "inspiring". When really overpowered you'll also feel that it is stiffer and actually start to miss the feel of the wood which offers a certain damping to the ride. But for you, given that you have the 66 I don't think you'll sail the 75 in rough enough conditions for this to be a deal.
In the end, I think the overall feel will be kind of similar to the 08, so if you didn't like that one the 70 might be a better board.
BTW, I checkup up the tank measured volumes and the 70 is 71 liters and the 66 (like you guessed) 64 liters. So still a reasonable difference, maybe.