We are going to Switzerland and and staying in the Stechelberg area. If we take the train to Lauterbrunnen the how do we get to Stechelberg? Would it be better to rent a car or to take the train?

There is a bus service from outside Lauterbrunnen station you can see it waiting as you exit the station. A car will be an expensive luxury spending most of the time in the garage in Lauterbrunnen and running up parking fees. Have you looked into the various passes available

Just take the bus. Having a car in this area is really of no use, as everywhere you'll want to go involves/requires cable cars/trains/boats. I guess the only reason to have a car is if you're staying in Stechelberg far enough away from a bus stop and you're continuing on to other areas in Europe that will require a car. Then I guess you could have the car to get close to your lodging and then just leave it sitting there when you're exploring within the area.