Brienz, Spiez or Thun in Mid-May?

Other than a drive-through, I'm not too familiar with these towns, but Brienz would be my preference, solely because this is where you can catch the Brienz Rothornbahn. However, it might not be open for the season yet in May...

...speaking of which, 10 days is a LONG time to spend in the Berner Oberland during the shoulder season. Most of the good hiking isn't accessible until June, at the absolute earliest. You might also find that not a lot is open during this time, as many of the businesses close for seasonal maintenance, or the owners take their vacation. Also, spring usually sees the worst weather in the Alps. You can hope for clear views, but expect far more overcast days than sunny.

In May, I would try to spend some time in lower-lying regions than the Berner Oberland, such as Appenzell (lots of rural charm) or Lake Geneva / Vevey area. Splitting your time between Brienz (not a bad choice) and that other location.

We liked Thun very much when we visited last year, though just for a day. It is not a village, but it is fairly tranquil and has a lovely lakeside, we walked from Thun to Oberhofen and the views were lovely. There is a nice square below the castle in Thun and, in fact, there are 4 castles in the area between Thun and Oberhofen. Quite atmospheric. Good bus and train connections, lots of shops.

We are staying in Thun for 4 nights beginning 9 May. I’ve been there before, and we have some family reasons for staying in that area, but find it beautiful with reasonable train access to other places we want to go.

Thanks everyone. Based on your answers, I'm wondering about going from Zurich around 11/12 May straight down to Ascona on Lake Maggiore for a week (to get warmth + Swiss feel + Italian colour!), then back up to Brienz area for 5/6 days in the last week of May, when hopefully the weather will have improved.
It has to be May, as we're already in Zurich for an event around 9th/10th.
From Brienz then we'd take the train back to Zurich-Paris-London.
Has anyone stayed in Ascona in mid May?
And has anyone taken boat trips on Brienz in late May + various funicular railways around there and experienced sunny days?!
Everyone is positive about Thun..but I feel drawn to Brienz's chocolate box charm....as long as there is eough to do.
Would you still persaude us against Brienz. It's 3,000 population compared to 42,000, but I'm guessing in late May it has a good vibe about it?

We stayed in Brienz the first two weeks of June 2007. We liked it so much that we returned later that year for another three weeks starting the last week of November. We had the Swiss Pass during our stays, so we were able to board trains, busses, as well as boats whenever we felt like it. We were never bored. But even just sitting in our lake-side apartment enjoying the views was great.

We found a lot of things to do while based there. We walked up the hill from our apartment in Brienz one day and discovered a zoo.