Going to Mexico soon. Will I get a better return by exchanging bucks for Pesos before I leave or while in Mehico? Does anybody have any suggestions? 

 From my experience: when possible, it is better to use credit card.  This will get you the best on-going rate.  And in Mexico's case, better not to carry cash around.  Better yet, unless you are going to Cancun or similar heavily fortified places specifically for foreign tourists, not to go to Mexico at all.  

I fear that if I were to use a CC, I'll end up spending twice as much. But you're probably right... 

It is very wise to use CC in Mexico. There is no extra charges for using your CC in Mexico. But make sure you call your bank and let them know that you will be traveling to Mexico.This will keep the bank from suspecting your account is being compromised when overseas activity is noted. And enjoy your stay in Mexico...Things are pretty cheap in Mexico.