Early march - Lauterbrunnen or head to France?

Mid-March is still very much winter and ski season in the Alps. Wengen and other towns of similar elevation will have plenty of snow. Hiking at those elevations will be impossible, but walks on the valley floor should be fine.

Swiss cities are under-rated and usually get ignored by visitors because the Alps in the summer are so awesome. I'd probably map out an itinerary focusing on them. If you'd still like a peak of the Alps, you could do a day trip to Lauterbrunnen easily from Bern.

Addie - Head to the South of France. Cote D'azur region. You will have better weather there instead of Lauterbrunnen. You will find plenty of outdoor activities in the South of France.

http://www.frenchriviera-tourism.com/out-and-about/cote-d%E2%80%99azur-events-schedule-06_2690.html

https://www.visitacity.com/en/french-riviera?gclid=EAIaIQobChMI5YevsrrH5QIVEp-fCh2emgCxEAMYAiAAEgIn8_D_BwE

You could stay in Switzerland easily in March yes it will be cold and snow in valley as Lauterbrunnen but it will be stunningly beautiful. Stay at least 2 nights in Lauterbrunen and during day take the quick gondola up to Murren to walk around half lunch and enjoy the views. Same with wengen take train trip up and have hot chocolate and pastry in a Swiss chalet restaurant so authentic and very Swiss. Also stay in Lucerne for sure. Lots to see and then take Swiss ferry boat up the river, ever so lovely.
Usually my first stop after arriving in Zurich airport is to take the train about an hour from Zurich airport station to Lindau, Germany on Lake Constance. A nice place to get over jet lag.
Your stay is quite short to spend so much time (2 days)in transit to and from Colmar, Alsace while these areas are lovely they warrant a stay on there own. I hope this helps. No one can predict rain or snow at this pt but u do have lots of nice choices.

You have 6 nights and I believe you can still have a good Swiss experience. If your flight arrives early, you can go right to Lauterbrunnen or on to Wengen or Mürren if you like. Stay 3 nights. There is still an opportunity for scenic walks. Our first trip to the area was in winter and we hiked on groomed trails, especially enjoying the Grütschalp to Mürren Walk (about 90 minutes) and there are some nice scenic walks in Wengen to Staubbachbänkii and Mönchsblick that you can do, each about an hour round trip. Combine walks with scenic rides on the mountain train, maybe even go to the Schilthorn and watch the skiers hurtle themselves down the slopes. That first trip for us spawned a love for the area and we go back often, though in the fall.

Then go to Luzern for 3 nights. I am not as familiar with the area there having visited only once but no doubt someone can join in with details. Luzern is close enough to Zurich that unless your flight is at dawn, you should be able to get to the airport without sacrificing a night.