My wife and I will be flying into Geneva mid Sept and have 4 night free on the front side of doing the Tour de Mont Blanc hike. We are thinking of spending those 4 night either in Luzern/Berner Oberland Switzerland or the Provence area of France. Would appreciate any thoughts, recommendations, pros & cons. Thanks.
Some quick thoughts:
Transit will be different for each of your options. If by "Provence" you mean the rural areas with small towns, lavender fields, etc, these are best seen by car. If you mean Arles, Avignon, Aix-en-Provence, Marseille, Nimes, and a few other places in this area, you will not need a car. Luzern and the Berner Oberland are great without a car (a car doesn't add anything to the visit). So, do you want a car, not want a car, or can you choose either?
Switzerland is more expensive than France.
Switzerland has mountains; Provence does not. Since you're hiking Mt. Blanc, do you want more mountains (even if it's just to look at rather than hike) or not?
The two areas are completely different. I'd go with Switzerland, but that's just personal preference.
These are so very different I think it can only be a matter of personal taste.
What I would do is construct a rough itinerary for both options, including sights to see, and decide which feels more compelling.
Possibly southern France has the edge given your subsequent hike--contrast. But you decide what feels right.
For Provence, consider a flight from Geneva to Marseilles upon arrival, to save time.