Buying a train ticket Paris to Switzerland (Bern)

Not sure what you are asking? Book a ticket from Paris to Bern with a reserved seat. Lots of choices.

My question is should I book in advance or can I wait until the day of travel.

Buy it in advance. France’s TGV trains are discounted up to 85% if bought 3-4 months In advance. The fare for tomorrow’s train, for example, is €150. During next week there are a few tix for €85. If bought for a date about 2 months from now, there are tickets for €65. Buying them when they first go on sale will give you the best deal. They often sell out and prices rise. France’s train company, SNCF, just made these deeply discounted tickets refundable and exchangeable within generous timeframes. They are called “Prem’s” or “Loisir” tickets and they include the mandatory seat reservation which usually is €20. You can
get a bargain ticket on the Paris to Bern route on a TGV Lyria train that takes 5.5 hours.
www.SNCF-connect.com

Also, the trains can sell out.

You can also buy the tickets from SBB, their website is a bit easier to use in my opinion. www.sbb.ch

Badger, So, is it the case that SBB will have tickets available on the same train for the same price as the SNCF Prem’s tickets ?

This is not the subway. You are taking high speed long-distance trains from one major city to another. I recommend you go online to purchase your tickets before your train or trains are sold out.

Badger, So, is it the case that SBB will have tickets available on the
same train for the same price as the SNCF Prem’s tickets ?

The TGVs between France and Switzerland are operated by SNCF and SBB together, using the brand TGV Lyria. So you should be able to buy the same ticket from SBB as you can buy from SNCF.

It's not only TGV trains that can be an issue . Last Saturday we took IC ( Inter cities ) from Toulouse to Carcassonne and return , on a day trip . Both trains were fully sold out , no seats without a reservation .

Tagging on to Kenko and Badger's posts above ,as an example - I just looked at my tickets for my trip tomorrow from Toulouse to Lyon . I purchased them months ago when they came on sale . 108 Euro for two . Only two seats available on my train for tomorrow at 250 Euro . The following two trains have been canceled , nothing further until later in the afternoon. Needless to say , two good reasons to book ahead , price and availability .