If going to France, the French side of GVA airport is super handy because you can rent cars from a French agency. For a one-way trip to drop-off elsewhere in France, this is fantastically helpful. They may look the same, but Hertz France is quite different from Hertz Switzerland, especially if dropping off in France. A one-day one-way rental to France may be $100 from the French side and some $400 from the Swiss side. Vive le French Sector of GVA!
There are four people in the OP's party. OP may find that renting a car from the French sector to drop off wherever in France may well be cheaper than 4 train tickets.