It's not that easy to try to guess which board will plane faster - Go with more volume or iS with more width? My guess is iSonic150, but what is often forgotten is which board is EASIER to plane? Here is what I mean: Go is longer and has more volume. If you're not experienced with new generation slaloms (or similar), you may find these boards hard to plane in the beginning. They are very short, very wide and need more sailor input to light up (feet positioning, weight distribution, pumping skills, etc). If you just stand there, you will lag behind a Go sailor.
Nothing to put you off, just be aware of it. Another question is Futura or iS? (easier, relaxed ride vs. sharper, more performance oriented) If you want to maximise lightwind performance (if you decide for iS or FE), you'll need a bigger rig than 9m.