Although I have never needed to have a bag authenticated from either source (knock on wood), I have heard nothing but excellent reviews about CarolDiva, but heard horror stories about MyPoupette. Not only that, I thought (and assumed) that MyPoupette only authenticates Louis Vuitton? Why did they authenticate your Balenciaga. Perhaps that's their downfall with this consensus, as well—they're not experts in Balenciaga, only LV.
Have you actually purchased the bag yet? Or is it something you're researching beforehand?
I would go with CarolDiva's advice, as stated. She has an excellent reputation and I would go with that.