See the title. The Nine are solely powerful entities within our Solar System dedicated to gravitational bodies, while the Witness and it's forces have destroyed thousands, if not millions of planets (gravitational bodies) across the universe. I just don't think the power levels in comparison are even close.
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In a direct fight the witness wins. But the nine see time differently and can plan ahead and nudge events in a way even the vex can't. The nine can also pull things forward to the present and affect the past and watch how the future changes. The nine are essentially playing chess at grandmaster level while the witness was playing at master level. The nine are by far more dangerous than the witness.