Both options will get you out of Geneva Airport. Which one suits you depends on four simple things.
How far are you going?
For a short hop into the city, either option works. The further you go, the more a price agreed in advance protects you from surprises, especially on mountain roads in winter.
How much are you carrying?
A rank taxi is the next car in line, whatever its size. If you travel with skis, a pushchair or several large cases, choosing the vehicle beforehand removes the guesswork.
Who is travelling?
Families who need child seats and groups who want to stay together are better served by a booking, because the seats and the vehicle size are prepared before you land.
When do you land?
Late at night or at peak ski weekends, knowing that someone is waiting for you is worth a lot.
Next step
If a booking suits you, enter your journey and compare vehicles, or read the arrival guide first.
