My wife and I have been to Europe several times to see Paris, Normandy Coast, Munich, Salzburg, etc. but my SIL, my son and his wife have not. I'm going to attempt to put together a trip for the very first time. Duration isn't as important and contend. Of course, I can travel East to West or West to East but I'd like my tour to include Paris and surrounding area, Normandy Coast, Zurich, (Innsbruck perhaps), Salzburg, Schloss Neuschwanstein Castle, Munich and Vienna. (I don't know why some tour company hasn't put this tour together before.) Anyway I'd LOVE to hear your ideas and comments, perhaps you'd know some very cool places along this tour that I should include (or not), how long I should plan to stay at each locale to see area sites, etc.. I'd like to do this tour any time between early April to October and as I said, I can't see any advantage to starting/ending at one place or the other. Thank you for your advice and experience!
Welcome to the forum!
Do you have non-stop service from either location to your home airport? That might figure in to my planning. I don't have non-stop service to any international destination but I can get to Paris with just 2 legs, lol. I like to start in Paris but that's because I love it. I could just as easily end there, lol.
Rick does have a Munich, Salzburg, Vienna tour if you are looking for an easy way to travel. In that event, I'd do the tour first starting in Munich, then backtrack to Switzerland and on to finish in France.
For Switzerland I'd consider being up in the Alps. A forum favorite and one of mine is the Lauterbrunnen Valley, staying up on one of the sides in Muerren or Wengen.
Time fo be spent at each destination would depend on the interests of your group, but Paris and Vienna are the major stops from the perspective of traditional sightseeing.
Mid-summer weather is likely to be noticeably cooler in Normandy and the mountains than in the other places you've listed which might affect the way I structured a trip such as this.
I am not knowledgeable about outdoor activities, especially in mountain areas, but I know folks warn about heading to the Alps too early in the year if you aren't looking for snow.
From recent personal experience I know Vienna can be quite hot in the middle of the summer.
In Normandy I highly recommend Monet's Garden in Giverny, Bayeux, American and Germany Cemeteries, and Mont St Michel.
As for Paris vs Vienna as your start or end points, As @Pam suggests, go with the best city for travel from the US. For me that would mean starting in Paris because I can go direct on my preferred airline. Vienna requires a connection through LHR which I perfer to do on my way home.
In Spring, Paris to Vienna. Each week later will be a better chance for good weather in Vienna. In Fall, the opposite. Weather will go downhill in Vienna before it cools down in Paris. Summer and Winter, I'd decide based on festivals you want to visit along the way (e.g. Christmas markets).