if I were you I would pay special attention to the people who have been there one or more times and have a personal basis for their opinion. Especially to Tim. I have lost track of how many times he has been to Muerren but it is a lot. His opinion is valuable.
Since 3 nights in the Lauterbrunnen area is the sum total of your time in Switzerland, you really don't need or want a car. Presumably you are going to the area to take advantage of the high mountain lifts like Schilthorn or Jungfrau. Why else go there? If you want to do a driving tour of Switzerland you would start with different plans. And if you are doing the Schilthorn and/ or the Jungfrau, you will probably want some kind of Swiss Pass or Half Fare Card to reduce the price. That will reduce the cost of train tix as well.
If you are just using your car to get four people to and from Lauterbrunnen you will pay for three days when you aren't using it, plus pay for three days of parking, unless you stay in Lauterbrunnen itself at a place with free parking. But why stay in the valley when the mountain villages are so much nicer? It is not a "schlepp" to get your luggage up, they put it is a big basket and carry it under the cablecar, then transport the basket by train to Muerren where you reclaim it. Easy-Peary and fun to watch. Or if you are going to Wengen you just get on the train with it and ride up.
I see from other posts that you are going to Milan next. So your car plan would require you to return to Zurich to turn in the car, and then fly to Milan? If you take the train to Lauterbrunnen then instead, you can just take the train from there to Milan. You will be there and n lesss time than it takes to drive to Zurich plus fly, and you won't have to pay for that day in the rental car.