What route to take in Switzerland?

Murren, Lauterbrunnen, or Wengen in the Berner Oberland is the place to be for a great alpine experience.

Yes. Murren and the Lauterbrunnen Valley is the prime location in all of Switzerland.

And PS: You'll love the Bernina Express. We did it southbound last summer. I wonder how different it will look northbound.

Thanks for your input! Any town preference or hotel preference?

[While in the Berner Oberland for 4-5 days]:

Should we spend the whole time in one town, or add another?

Choose a hotel in one of the towns. They're all in or above the same valley, so you'll visit or pass through all of them as you explore that area.
Lauterbrunnen is at the base of the Lauterbrunnen Valley, where the train line ends coming from Interlaken Ost. It's at the bottom of a cliff with Staubach Falls, and many other waterfalls along that valley.
Mürren is the town up a mountain on one side of Lauterbrunnen Valley and has the most spectacular alpine view. It's on the cable car line up to the Schilthorn.
Wengen is the town up a mountain on the other side of Lauterbrunnen Valley and has a different gorgeous alpine view. It's on the cog rail train line up to the Jungfraujoch.

Here's a panoramic map of the Berner Oberland so you can see how the towns are situated (you may have to enlarge the picture):
http://www.regionalpass-berneroberland.ch/assets/karte-und-partner/Regionalpass-Panoramakarte-2017.pdf

And here's a Swiss travel system map of the whole country:
https://www.sbb.ch/content/dam/sbb/de/infotexte/uebersichtskarte-sts.pdf

Thank you Linda, that was really helpful! Have you been to Grindelwald or Jungfraujoch? Wondering if they are worth a day or if the whole time should be spent in the Laut.valley?

My preference is always to stay up on the mountains rather than in the valley. Grindelwald is in the valley beneath the wall of the Eiger, sometimes in its shadow. Muerren and Wengen are up higher, get more sun (when its shining), closer to the hiking trails and lifts. It is a higher rent district as well. Worth it in my book.

Have you been to Grindelwald
or Jungfraujoch? Wondering if they are worth a day or if the whole
time should be spent in the Laut.valley?

Grindelwald is in the next valley, and is at the base of a cable car ride up to "First" which has a different set of spectacular views. Yes, I've been there. What I like about Grindelwald is that it's on the way to "First" and from First I take a short hike to the Bachalpsee--a little lake high up in the mountains which reflects the snow capped mountains when the air is still. I also like the beautiful view from the First cafe terrace and its cantilevered viewing platform, as well as the cliff walk there.

I've enjoyed looking at the Jungfrau mountain from Kleine Scheidegg, below, but I haven't been to the top because I get altitude sickness that high up (Jungfraujoch is at 11,371 feet). (Kleine Scheidegg is only 6762 feet in elevation.) I enjoy the view of the Jungfrau while hiking from Männlichen to Kleine Scheidegg, and from Kleine Scheidegg to Wengernalp. (Those hikes can be accessed from either Wengen or Grindelwald, but Wengen is closer.)

Mürren provides access to my very favorite hike, called the Northface Trail, via Allmendhubel, a funicular ride above Mürren. And I hear Schilthorn is spectacular, a cable car ride above Mürren, but I haven't gone up to Schilthorn yet.

I think you should go where ever the sky looks clearest. There's a tendency out there for a thick blanket of cloud to roll in over a mountain and obstruct your view, especially later in the day.