There is a lot of debate on this one but for me the French-side swings it. It is located on site (no need to wait for a bus – albeit its only a 5 min trip); is usually a good deal cheaper but is a little harder to return to than the Swiss drop-off (see below).
One thing you need to be aware of when driving from Geneva airport to is that if you choose to go on the Motorway (and it is a lot easier), a “Vignette” sticker should be displayed in the window of the car. It can be purchased at the border, costs around £30 and apparently lasts a year (all the good that will do you since you’ll only be on the Swiss motorway for around 15 mins). You can risk not getting one but there’s a 80% chance you’ll be stopped on the way back in – which might slow you down when you’re rushing for a flight.
Cars hired on the Swiss-side usually have a Vignette sticker already and so the £30 difference may make the Swiss prices more attractive (Swiss cars also come with winter tyres in winter).
It is possible to avoid using the motorway by driving through Geneva (you can find the directions from the French side here – thanks Ma for the scans) but it will take a little longer and there’s a good chance of getting lost).
The biggest hurdle to hiring from the French side is the return. If you mistakenly end up at the Swiss drop off (as we once did in panic) you may be charged a lot by the rental company (Sixt didn’t but I think Hertz do)
The little rat run from the hire location to the road drops you right at the border crossing in an area known as Ferney-Voltaire. So look for directions to here when you are returning. You head past the airport and take an exit which also has indications to the “Palexpo” area. The GPS location for entrance to the rat run is N 46° 15.024 E 6° 07.081
Once you’ve dropped your keys off on your return, head for the Swiss Side of the airport via the entrance to the right of the one you entered the French side when you arrived. Go past the luggage carousel and take the door on the left, double back along the corridor and head down the escalator. Head straight through the duty free area and you’ll find yourself in the Swiss baggage reclaim area where you arrived (yes it’s odd).
Head through the Green customs exit and repeat the path you took when you arrived at Geneva – turning left in the arrivals hall and heading for the first escalator or lift on the left. Go up one floor to the check-in area and you’ll find Swiss Air is straight ahead of you, EasyJet as far as you can go when you turn left and BA somewhere in between.
Something else worth mentioning is that the Swiss Customs have turned tax collector and are now pretty much stopping everyone coming back in from France to see if they have a valid Vignette and that it is properly stuck to the window. If they stop you and you don’t have one, feign ignorance and they’ll send you to queue in the office to buy one. So, worth building in an extra 15 mins
This is the Google maps link to the road that returns to the French car hire
https://goo.gl/maps/m37huTC4xgUXFTdC9
Something else to factor in on car hire is that cars on the Swiss side tend to come with all-season tyresand so can drive to the mountains in winter without needing chains on board. Those on the french side tend not to, so you’d need to hire chains in winter (it’s the law to carry in the mountains)