Which Train pass to buy for a trip of 8 days in France and switzerland

so are these the only two places you are going now? No Spain or England? Germany, Italy?

I would not get a pass for France. Buy discounted advance purchase nonrefundable tickets once your schedule is set. Switzerland you should consider a pass of some sort, again depending on your schedule.
You can't go wrong with the Berner Oberland: Lauterbrunnen, Muerren, or Wengen. The transport infrastructure is outstanding and scenic beauty is out of this world. There is a lot more to Switzerland and the Alps than the B.O., but for a first timer, it's a can't miss.

The planned route of the trip is key to comparing costs, and if other countries are involved, then they should also be considered. For instance, 8 days in two countries might only require one train from Paris to a Swiss home base such as Muerren in the Berner Oberland, local rides of various duration, and then out to an airport such as Zurich. For that scenario, you could look at an advance-discount TGV ticket from Paris to Basel, then a Swiss Transfer Ticket + Half-Fare Card, or a Swiss Travel Pass for 3 or 4 consecutive days, or just a Half-Fare Card. Or, maybe you plan something quite different. See more about Swiss passes at http://www.ricksteves.com/travel-tips/transportation/trains/switzerland-rail-passes.

Gems,

You may want to read info on this website before deciding if a train pass is worthwhile for you: http://www.seat61.com/Railpass-and-Eurail-pass-guide.htm#Should%20you%20buy%20a%20railpass%20or%20pay-as-you-go

Depending on where you will be traveling & how many days you will actually be traveling, it may be more economical to buy point-to- point tickets, especially if you buy them in advance at a discounted price.

This is the website for train travel in France: http://www.sncf.com

and this is the website for the Swiss Railways: http://www.sbb.ch/en/home.html

You may also want to search this website for train schedules within Europe: http://www.bahn.com