rlewysohn:
Responding to your questions:
I'm targeting LBS MBA or UCLA MBA so I'm not sure if I should cancel and retake, only ratake, or cancel and use the 650 for the admissions process.
--> You should keep and retake the exam given your practice range. There's no harm in taking the exam a third time, especially for potential applicants such as yourself who have mixed results between higher overall scores on one exam and lower overall scores with a higher relative quant score on another.
Admissions take into consideration that eventhough in the 650 test I score poorly in Quant, in the other test is acceptable?
--> This is tricky. They will count your overall highest score, so in this case the 650. They will see your 620 (including your Verbal score) so they will acknowledge both your lower overall floor and your quant ceiling. How they consider it - that's anyone's guess. It's possible they don't really even acknowledge it. All that said, the 650 is your best bet - a higher overall score trumps a better individual performance on one section.
Do they take also into considerations that I have the CFA Charter (that can be thought has a proof of quantitative knowledge?
--> Yes, but it's not a one for one substitute with the GMAT score. This is secondary in terms of importance.
Overall, take the exam again. LBS will want as close to a 700 as possible. UCLA might be more forgiving but not so much so that it's worth resting on your current performance.
Good luck,
Admissionado