I have been both an event planner (designing and implementing the concept from scratch) and working with existing events in business development or sponsorships, for events during major film festivals and others.

The answer to you question really depends on a few factors:
- have you worked with this festival before? Do you trust them?
- Are they funded or are they seeking funding?
- Have you discussed amounts for the fee or percentage?
I personally would suggest a hybrid of reduced fee plus percent if it's reasonable that you should share in the success of the festival.
To do it on percentage alone has a risk of not materializing if for some reason the festival does not get funded or doesn't go ahead - then you will have done the planning work, yet receive nothing.