I do not know zurich, except yes it is very expensive. I know what postdocs at ETH earn and what those in Bern earn, and the difference does not make up for the raised Zurich prices.

One thing, I am not sure if you would be on local contracts (if you will be paid by the EU let me know, as you need extra advice), but settle your salary before you sign anything.

I am at Bern uni, and am saying, you do not want surprises after having relocated. One thing, I hope you are a high level academic, otherwise I do not see how you can support a family by yourself at ETH. But perhaps ask a few scientists there.....And if you depend on your wife getting a job, be careful, this is not a guaranteed situation (the partner getting a job); other expats suggest this may be more difficult than in other countries.

Oh, and never settle for just a gross salary, ask ETH to send you a list of your deductions, so you know your netto salary after taxes. To give you an idea, I pay 37% of my postdoc salary in taxes/deductions.