[ryan elder] “Okay thanks, but in order to send it into festivals, I have to know the pixel aspect ratio to export it in first, don’t I?”
You don’t even have a film to export it. Shoot your film first, get accepted into a film festival, then ask this question.
[ryan elder] “Well okay then, I will film it in either 2.20:1 or 2.39:1 and then ask what the pixel aspect ratio should be afterwards.”
This is not a question you have to concern yourself with.