I have just moved to Gva from Zch, having spent 5 months in the latter.
I'm only in Gva a week so take my comments with a pinch of salt.

Cost of living is similar I think so far...
Culture - definite difference i.e. French vs German.
Zurich seems better for nightlife and that's the general consensus. I had a good time going out in Zch.
Can't really judge too much but I thought Zch was really nice and concentrated, easy to hop around between diff areas, whereas Gva seems more spread out.

From what I hear Gva has better choice of ski resorts and mountains, eg France, but there are some good ones near Zch - i.e. Davos, St Moritz, and there's a couple of decent ones an hour away....

Can't really judge the people yet... Zch'ers were ok I thought once you break them down! But hard work getting htere. Having said that my Deutsch skills are crap.

I really enjoyed Zurich but so far I'm impressed with Geneva. I'd say if you liked Gva then you'll like Zch - but there is a distinct cultural difference.

I will be able to add more later of course....!