Hi Joel,
First off, 640/650 are strong scores (they're right around the 80th percentile overall), so either could be enough to get you into your first-choice School. As such, a retest might not be necessary. You're ultimately asking Admissions questions though, so you would likely find it beneficial to speak with an Admissions Expert about your overall profile. There's a Forum full of them here:
ask-admission-consultants-124/Many Business Schools place a higher value on an applicant's Quant Scaled Score, so if choosing between these two scores, it would likely be more beneficial to your overall application to submit the 640.
1) What Schools do you want to apply to?
2) When are you planning to apply?
GMAT assassins aren't born, they're made,
Rich