June vs September--Berner Oberland?

It is hard to say, weather in Switzerland, and especially in the Alps, is unpredictable.
I would still lean towards June: the days are much longer. I also think, but I am not sure, that September can be more prone to thunderstorms.

FOR HIKING--Early June you can still have snow on some trails but I have had also had snow 4th of July as well--agree unpredictable

Early September is very nice and my preference--less crowded with kids back in school-but my mid September weather turns

My only time there was in mid-September 20 years ago and experienced two days of intermittent drizzle. This is not unique to the time of year and similar weather can occur in June as well. It will be a luck of the draw. However, I still was able to hike and see most of the sights I wanted.

We always go in September, usually mid-month for two to four weeks. With a long stay, a day of drizzle is no problem.

Last year we went in late August, stayed a month, and I have to say we loved the weather! June would worry me in terms of snow on high trails although this year, right now, they are having a heatwave!

Been there a few times in mid to late June and had great weather. Never been in September.

It can go either way in the mountains, but personally I prefer September for hiking/walking as it's usually less humid (and less chance of rain) and there are normally fewer people. Last year we were hiking in shorts and sweatshirts into early October, with brilliant crisp blue skies, while in June it was pretty muggy and the views were sometimes hazy. A couple of years back, my parents came to visit in June and we had thunderstorms every other evening, but perfect days in between.
Both June and Sept are good times of year for a trip and you never know what the weather will do anyway, so I wouldn't stress too much about it!

I've recently done both--sept last year and june this year. Preference is Sept. Would have liked to have done Sept again this year but didn't want to wait a whole year to get back. :) That being said, either is a great option. Do what works best for you.