If you go for the Isonic you will need to dedicate some time to getting it dialed. They are more technical to get planing and go fast than the average freeride board. I'm new to Isonic but I did find that I needed to point the board downwind more than usual to pump it efficiently on to the plane. When you get it right is fast and efficient. You may want to see if you can demo it to make sure you are happy with the type of ride it offers.
My 7.8m is great match for the IS111, and I also use 9m which works surprisingly well. For me the board is not as happy with my 6.7.