Wear either. Or dress down. If you pick that one seat in the fourth row, dress up. Everybody around you is there to be seen. If you're on the fourth balcony in the cheap seats, dress down. Everybody around you is there for the opera. In general, fancy on the main floor and first balcony. Above that in the nosebleed sections, there's less worry about what you wear.

In general, German speaking areas put on better German opera. They're pickier about it, and almost all the composers are local or almost local. You screw up the accent and the notes at your own risk. French, Italian, Russian, etc, you've got a bit more leeway.

Zurich ustta be a bit of a burgerlich stadt where the money stayed with the gnomes...