Particularly because you were scoring so well on practice tests, I recommend you retake the GMAT. Remember that your highest GMAT score is self-reported on your application, so admissions teams will focus on your best score. There isn't much harm in taking the test again. If you are very interested in Columbia and NYU Stern, I recommend you aim for a 700 on the exam.
If you decide to take the test again, here is a free GMAT overview: http://stacyblackman.com/gmat-overview
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Stacy Blackman | Stacy Blackman Consulting Inc | http://www.StacyBlackman.com | +1 323.934.3936
MBA blogger, US News and Author, The MBA Application Roadmap