Hello,
It's a tough call... I think the question you have to ask yourself is how badly do you want to get into a school like Booth? If you really have your heart set on Booth, then the more you can improve your score, the better. That said, I'm not sure that a 660 would move the needle there, so if you think that's the highest you could score, it might not be worth it (at least for Booth).
For your other schools, though, it seems like a 660+ score could definitely increase your chances if the 640 is the bottom of the 80th percentile range. If you truly believe you can do better, then I'd say go ahead and retake the exam as soon as possible and get it out of the way. Then focus fully on your applications. Don't worry so much about R1 vs R2 (though if R1 is November for your schools, you've got plenty of time to do both) - worst case scenario you apply in R2 with higher quality applications! There is not a substantive difference in acceptance rates between R1 and R2, generally speaking.
Hope this helps,
Mili
Mili Mittal
Senior Consultant
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