The reason why there is a separate option that allows you to just reserve the seat, without buying the actual ticket, is because some of us locals have the AG or (GA in German), which is a year pass that covers all public transports within the country and discounts on privately operated routes. It turns out the Glacier Express accepts the AG so if I wanted to ride on it, I just need to pay the seat reservation and that's it. The rest is included in my year pass.

I've done the Glacier Express once, from Brig to St Moritz. I would skip the Zermatt-Brig portion, nothin really special, unless you really wanna see the Matterhorn quickly before getting on the train. The interesting parts that I liked were the mountain passes, they offered great scenic views.
Keep in mind that all this is weather dependent... if you're unlucky and it's raining and cloudy the day you travel, you won't be seeing much....

If you're based in Zurich, why not take the Express the other way round? Hopping on the train in Chur or St-Moritz and go all the way to Brig or Zermatt. Maybe all the way to Zermatt and spend a day or two there so you can go up with the local train to get views of the Matterhorn?