I don't think the sails matter much, if anything the more compact sails would fit better on the Q76, but in practice I think it will depend much more on what kind of lines you want to draw with the boards. The 81 has a tad more drawn out rocker and outline and at least for a heavier rider I think it would be slightly drivier in big sideshore stuff. But for me the 76 is DEFINITELY drivey enough and I love the more compact feel of the 76.
But it will depend a bit on your weight also. If you weight under 75-80, the 76 will easily handle 5.3 and then I don't think you will notice the smaller volume when going out. You still have the width and most of the surface area. Only if you sail a lot of underpowered sailing I think the 81 might be something to consider.
If you are closer to 70 (like me) the 76 is definitely the way to go. If you're over 80 further considerations need to be done.