I find Geneva lovely, if a bit more sleepy than Zürich. That's not a bad thing, just personal preferences.
I find Geneva great if you want easy access to Lausanne or Montreaux, or to anywhere in the western Swiss Alps. Trains from the airport are frequent.
And Geneva is great if you want easy access to the French Alps - it's my typical gateway airport for riding my bicycle on the epic Tour de France cols, given its ease of connection by train, bus, or car (or, indeed, by bicycle).
Zürich is better if you want easy access to the Bernese Oberland or to any point in the eastern part of the country. It is a more "modern" city, if you will, with a lot more going on - and has a bit more bustle as a result. The airport isn't as convenient to the city, proper, but it's not much further away.
Flip a coin - you can't really go wrong.