Hi Alex,
To start, your question is a bit 'biased' because you're not comparing 'like' things (a score in the Quant section of the GRE vs. the OVERALL Score in the GMAT). Does the Master's Program that interests you care AT ALL about how you perform on the Verbal section of the GRE? If not, then you would likely find that taking the GRE would be the better option (assuming that your overall Quant skills are strong).
Before I can offer you any additional advice, it would help if you could provide a bit more information on how you've been studying and your goals:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)? Have you taken any practice GREs yet?
4) What application deadlines are you facing?
GMAT assassins aren't born, they're made,
Rich