Passing data from a notifing function to a listening function

What are the trade-offs for...

1. Using a custom event.EventData class to pass data from the notifying function to the listening function

2. Using the source argument returned in the listening function to access the data directly

Specifically, a model/state class generates a large data set in a while loop. At the end of each loop, an update event notifies a plotting class of the desire to plot the data. Which data exchange mechanism would have the lest effect of the performance of the while loop.

Thank you