Switzerland for 8 days/7 nights

Whether you split your time between the two places depends on your primary goal. If you really want to hike and experience the Alps, then base your time in the Berner Oberland and take a day trip to Luzern. It is 2 1/2 hours by train from Lauterbrunnen, very easy to do as a day trip. I personally would spend the whole time in the Alps, because you are likely to get a couple days of cloudy or rainy weather. You can use one of those days for a day trip to Luzern. Murren is a fantastic little town. It is just big enough that it has the stuff you need, like a COOP for food and little restaurants. Gimmelwald is cool, but it is really small. There is a cable car connection between the two villages that is quick and easy and frequent, or you can walk 30 minutes. The downhill walk from Murren to Gimmelwald is much easier than the walk back up.

Stay in Gimmelwald if you want a wonderfully bucolic atmoshphere, Murren if you want easier access to restaurants, atms, groceries, laundry and other conveniences. I stayed in Murren and hiked to Gimmelwald to spend the day wandering. It is only a mile or so. It seemed like a good blend.

When are you going? Mürren and Gimmelwald are less than a mile apart, but Mürren is up higher. You can choose bases on what type of lodging you prefer. Either one makes a fine base for hiking. Luzern is expensive and more of a city experience. One or two nights might be enough there.

Unless you are going only to hike, I would split the time roughly as you have suggested. As you express interest in Luzern, I would think twice or three times about Gimmelwald. To call it a hamlet is an exaggeration. There is absolutely nothing there and has none of the charm of Murren or Wengen. Luzern is a wonderful city and there are side trips including a steamer cruise on the lake. Luzern is much more expensive, if that is a consideration. Yes, weather is always an issue, but I don't think I would sacrifice other experiences in order to buy a better chance at weather. Four days is sufficient for the BO.

Logistically speaking I think Murren has an advantage since it has a Coop store, you can easily hop on the train that takes you to the lift back down to Lauterbrunnen, and it's also the lift point to the Schilthorn. You can easily take the wonderful walk down to Gimmelwald as opposed to up to Murren, but there is a Murren-Gimmelwald lift anyway. Gimmelwald is more secluded-feeling with a tiny village atmosphere. I've never been to Lucerne, so I don't know how cool it is. I just know I wished we had more than the three nights we stayed in Murren.

I split my week between the wonderful B&B in Gimmelwald and a small apartment in Murren - perfect. Got the tiny G'wald ambiance, and the incredible views and ease of access to lifts/train,etc. of Murren. Loved it!