Hi gDubs,
That is a tough question! Well first, what you were told is correct, Anderson is really strict about not receiving updates after you've submitted. Other schools will allow updates though, so it's definitely worth checking with each school if you applied to others.
Personally I think the jump from 660 to 680 is significant, and a higher overall score is better even if your Quant dropped slightly. I actually polled a few of my
mbaMission colleagues to see what they thought of your question and most thought it was worth withdrawing and resubmitting in Round 2 with the higher GMAT. And then if you do that, write a very short optional essay pointing to the higher Quant score on the previous GMAT attempt.
So just based on what I know, that's what I'd vote for. But I think it could depend on other factors...like how strong is your GPA or other academic indicators? Are you applying from an over-represented demographic? These are some other questions to consider. Feel free to PM me with more details if you want to discuss further.
Good luck!