Both are great areas with lots of positives - lots of aires/stellplatz, fabulous scenery, good drinking of beer and wine in both (we were very surprised by the wine in Bavaria - local stuff much nicer than any they export to us). On negatives - we've had rubbish weather in both places, but equally we've had great weather. We were in Brittany a few weeks ago and had mixed weather, but still managed to have a good time.
Personally I'd leave the decision as late as possible and keep an eye on the met office.
Lesley