Paris to Zurich or Bern?

You get the TGV to Basel, then connecting train to Interlaken Ost, then connecting trains on to the mountain towns of the Berner Oberland. Look up the route and train options at www.sbb.ch/en.
Then to Zermatt, then to Milan. Easily done.

when are you going?

There are significant troubles travelling in France at the moment, until at least next year...

Probably late October/early November 2020.

What I need help with is getting the TGV to Zurich or Bern. We would optimally like to see the Bernese Oberland area then head south to Zermatt/Matterhorn. Where should we head in to? Zurich or Bern?

mbeaulieu, I will say what I always say, you are putting the cart before the horse. If you want to stay somewhere in the Berner Oberland, first decide where, then look up trains from Paris to wherever you have chosen. This will tell you the quickest route, which may be via Zürich, Bern or neither.

The best part of the Berner Oberland is the Jungfrau Area (Mürren, Wengen Lauterbrunnen or Grindelwald). You can find out all about this area here (official tourist website): https://jungfrauregion.swiss/en/summer/

You can either do this on its own, or with Zermatt: Paris - Jungfrau Area - Zermatt - fly home
Or: Paris - Zermatt - Jungfrau Area - fly home
Or: Paris - Jungfrau Area - fly home
All by train of course

The nearest airport is Zürich, a lot quicker than going to Milan. There are trains direct to Zürich airport (and Geneva airport)

Now, to look up trains. Go to the SBB (Swiss Railways) website: https://www.sbb.ch/en
And look up Paris to wherever in Switzerland, add a date and time. Times for most of 2020 should already be posted, but you can only buy tickets 3 months in advance. You will see approx 1 connection every 2 hours.
For example, Paris to Lauterbrunnen, starting at 08:00 on a random date in May:

Paris Gare de Lyon dep 08:18 6 h 37 min 4 changes, Lauterbrunnen arr 14:55
Paris Gare de Lyon dep 08:18 7 h 7 min 3 changes, Lauterbrunnen arr 15:25
Paris-Est dep 09:28 6 h 27 min 3 changes, Lauterbrunnen arr 15:55
Paris Gare de Lyon dep 10:22 6 h 3 min 2 changes, Lauterbrunnen arr 16:25
Paris-Est dep 10:55 6 h 0 min 4 changes, Lauterbrunnen arr 16:55
Paris-Est dep 10:55 6 h 30 min 3 changes, Lauterbrunnen arr 17:25

And more trains after that. Those trains are not all the same route, some via Geneva and some via Basel.

Now all you have to do is decide where you want to stay!

Edit: For info on the trains from Paris to Switzerland, where and when to buy tickets, see here: https://www.seat61.com/lyria.htm