I have been to both places. Recently to Grindelwald but probably 15 years ago to Zermatt.
We go to Switzerland for the hiking and have spent a week or two in Grindelwald several times. The hiking there (easy hiking) is very good. I have a
Day Trip describing the hiking in Grindelwald. What I like about the hiking in Grindelwald is that there are many mostly flat hikes because you take a ride or bus up the mountain, then walk across to another ride or bus down. If you want to do train trips and explore the area, there is a lot to see in the Grindelwald area. There are other mountain towns - Wengen, Lauterbrunnen, Murren. You can easily get down to Interlaken and the lakes. Meiringen is not far. So you could do some easy hikes and then do some exploring.
But, Grindelwald can be busy. The Japanese tourists love Grindelwald. Sometimes the local train is crowded with people going up and down to the Jungfrau (ride up to the glacier). It is not busy on the trails - you see other people but it is not crowded. The town is charming. And you are right in the mountians with incredible views. I would happily spend two weeks in Grindelwald.
The other "but" for Grindelwald is that the mountian rides are expensive. Go on their website and see the prices for mountain ride passes. Or you can get discount cards - like the half price card (you pay 125CH for it and trains are half price but some mountain rides only 25% discount). There is also a Swiss Card and a card for the Berner Oberland. All worth looking into.
Early to mid-September is a perfect time for Switzerland. Sunny and warm. We were in nearby Lenk this September and the weather was very good. August can get some afternoon rain storms, but it still good.
We spent two weeks in Zermatt 30 years ago and have returned only once I think. For us the hiking is not as good there. The mountain rides go up very high and there are not options for easy walks (except one - walking back from Zermatt towards the Matterhorn - there are nice walks out there). But, the town is beautiful and the location stunning. It is not as easy to get back down to the valley to explore other places. You can't bring your car into Zermatt.
I will tag
@kay perigo who was in Zermatt and Lauterbrunnen the summer to see if she has something to add.
Which mountains do you live in now? Switzerland is very different from mountain areas in the US and Canada. It is a different experience.