Well the Simplon is about 15-20 minutes of a 4-4 1/2 hour journey. I would guess both are rather good train rides, probably in the top ten train rides in Europe.
For Milan - Geneva, you would get to see Lago Maggiore, Stresa, which are very beautiful, then the climb up to the Simplon on the Italian side, which is quite spectacular, then the ride down the Rhone in the Valais on the Swiss side, followed by the length of Lake Geneva. There's something to see out of the window, pretty much the whole length of the trip.
Milan - Zurich will be a bit similar, though I'm not familiar with that routing.