The Vex are certainly more powerful, what with the whole warping across galaxies thing.
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The geth are controlled by reapers who probly shit the vex out on accident.
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The power of the Reapers is irrelevant to the question. The Geth were created by the Quarians, and are controlled by a group consensus, independent of the Reapers. In the events of ME2 we learn that the group has been divided in two: those that worship the Reapers as gods, and those who do not. Harbinger himself doesn't seem overly impressed with the Geth judging by this quote "Geth, an annoyance... limited utility."
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You will know pain Shepard