Trying to safe a buck after spending more on a one-day guided tour of Rome! What's your opinion of the 4 hr Flixbus?
If traveling solo or in couple with no children, Flixbus is a great option to conserve money considering how bloody expensive Swiss Rail is! I imagine that would be pretty scenic ride a great value; as I took Flixbus from Munich to Salzburg and it was 5 euros.
Thing is, there are special advance prices as low as 35 CHF for Eurocity Mini fares for Zurich to Milan by train. 3 hours and 26 minutes.
Looks like mini fares must be purchased 21+ days in advance... the bus it is!
Just be ready for the very long (10.5 miles) Gotthard Tunnel. The bus will go through there, not over the Alps.
A side issue. What is your full itinerary? Why are you in Zürich in the first place?
Most people flying into Zürich airport go directly to the Swiss Alps, going directly to Milan doesn't make sense as you can fly directly to Milan.
If you are staying in Switzerland, Zürich is not the place to stay, it is not in the Alps.
Take a nap when it goes through the tunnel.