Hi Vanessa,
Porta is a great company, and I used Movistar last year and had full coverage everywhere I went. I believe the prices are similar too. However, I think I remember noticing more places to buy more minutes from Porta than for Movistar in the area where we'll be living... so keep that in mind.
As far as where to buy cards, I'm almost positive that they do sell them at the airport, but they also sell them at the Supermaxi, a grocery/drug store, that is at the top of a hill in the Mariscal in Quito, where you'll be staying before going up to Cangahua. Any staff member who has been on the project before will be able to help you get there if you can't get a card at the airport. The Supermaxi is also a great place to get anything you forget to bring with you, such as a toothbrush, sunscreen, aloe, towel, razor, etc.
Hope that helped,
Micah