Would you rather stay in Avignon for an extra day (for a total of 2 days) or go to Lyon for a day before going back to Paris to fly out (and therefore only have one full day in Avignon)?
I would probably spend the extra day in Lyon. But are you planning to go to Arles? That would actually be my top pick for an extra day! We stayed in Arles & took a day trip to Avignon. I don't remember how far apart they are, but close enough for a day trip -- so not too far.
I agree with Karin's comment about Arles. (In fact, I would stay there over Avignon.) But here is something else to take into account.
You can get a direct TGV train from Lyon (or for that matter from Avignon) to Charles de Gaulle. The trip from Lyon is 2 hours. Depending on your flight's departure time, you may not need to go back to Paris at all (unless you want to), potentially saving you more time that you could spend in Provence or Lyon.
If this interests you, make sure to snag the direct train, as many go to Gare Lyon in Paris.
I would opt for Arles then Avignon (half day) over Lyon.