The Go has 30 more liters of volume, but it is about 4kg heavier than the iSonic 150. Widths are only 2cm different with the IS being wider.
The is just a calculated guess, but I would say that the IS will plane quicker. The lighter board will be much more responsive to gusts of wind and will be easier to pump on plane.
Given the size of the IS and it's relatively light weight, it will be a handfull in 20 knots. The IS boards are fast and can be pretty scarry unless your skills are pretty good. I have an IS 111 and I am on either a 6.6 or 7.5 in 20 knots. I have hit 31 knots of board speed in these conditions.
One thing not mentioned before is that the iS boards have the foot straps pretty far out on the rails, like the Formula boards. It's tricky getting into the footstraps and it takes a lot of pressure/force on your back foot against the fin to keep the board flat and moving. This is one reason the Futura is easier to sail.
I haven't sailed a large IS or a GO, so hopefully someone from the "team" will offer some more advice.