What city to stay in Geneva or Montreux

While Geneva itself is fairly interesting, it is far from the main attractions of lake Geneva. Montreux or nearby Vevey are much better bases!
Afterwards, to get to Paris, I encourage you to take one of the TGV from Lausanne taking 3hr40min or so (for example at 12:23 on weekdays). It is a bit more scenic and a bit faster than the route via Geneva. But if you end up taking one of the more frequent connections via Geneva, it is still a nice journey.

I agree about not staying in Geneva. We just returned from a trip where we stayed in Lausanne (on the lake) for 4 days after we flew into Geneva. Lausanne is worth a day trip; we would have preferred a smaller town per the above comment. We did Rick’s walking tour of Lausanne the day we arrived. We did days trips the next 3 days. The 4th day if rained like crazy so we had a down day and toured the Olympic museum which was very close to our hotel.

I like Montreux so much I’m going back next year. My trip report might give you some ideas for the area and I mention my hotel:

https://community.ricksteves.com/travel-forum/trip-reports/solo-in-switzerland-my-trip-report

I also highly recommend Montreux; we didn't get as far as Geneva but there's some great sites in and around Montreux.....such as the Queen museum, Chateau de Chillon, and the beautiful promenade along the water. Lausanne and Vevey are very close via bus/train as well. We also went to Ouchy and took a 30-minute boat ride to Evian France for lunch! We loved Montreux so much we are returning in 2024.

Vevey--spent 3 terrific days there in Sept '23. Can't recommend it high enough. Walkable, friendly, accessible via Schweiz train to anywhere near and far. Right on Lake Geneva.

Thank you for all the comments and suggestions. Really appreciate it!
Joan