Hi tchwang90,
First off, a 680 is a strong score (it's right around the 85th percentile overall), so it could be enough to get you into your first-choice School. As such, another retest might not be necessary. Many Business Schools prefer that applicants have a high Quant Scaled Score, so you might be better off submitting your second Official Score (with the Q47). 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/GMAT assassins aren't born, they're made,
Rich