Belgium or Provence, France?

You can look to the weather. Temperatures in August in Belgium usually just above 20 °C, quite pleasant if you ask me. In Provence the weather will be considerably hotter.

Thank you! That is one point to consider.

There is so much more to Provence than Avignon. I loved the history that you see there, but it wasn't until I went into the countryside and to the smaller towns that I realized how much the area has to offer. So, do a separate trip for Avignon and include other parts of Provence. Belgium is also closer and so there will be less travel time.

The huge arts festival in Avignon ends July 25. I wonder if anyone closes to recuperate? Remember that August is vacation month in Europe. The two destinations are not comparable, but I agree it can be really hot in Avignon. It would be important to know if you plan/wish/hate to have a car.

I don't see the value in the long-ish train ride just to visit [Avignon,] a swell city but with no car to see Provence. OTOH, Belgium is perfect to see four to six important towns with only local, day-of-travel rail tickets. (Okay, you can't do that much in three nights!) Can you still fly home from Brussels? Warning: I don't think it is that easy to train to Bruges from Paris-there would be changes. I would start in Antwerp (some direct trains from Paris), nearly as nice as Avignon, but very different and more urban. (Other posters would choose another city to start in Belgium.)

Thank you so much for the advise. I think we will just go ahead and visit Belgium since we have so little time. Will save Provence for another trip!