Swiss Travel Pass 1st vs 2nd class

No difference I am aware of.

First class cars are often less crowded and seem to have plug outlets more frequently-- I'd save my money and travel 2nd, though-- just as convenient, and I only once or twice struggled to find a seat on a crowded train.

The only difference is that, on trains with two classes, the 1st class pass gets you into the first class seats. Remember that not all trains have two classes. On the other hand, for some of the named scenic routes (such as Glacier Express or Bernina Express) the first class pass gets you seating in prime seats in the panoramic cars (details in Rick's book). However, the windows in those cars don't open, so your pictures will have more reflections.

In terms of transit itself, both 1st and 2nd class passes cover 100% of most travel in the country (including trains, local buses, lifts, and boats); 50% of a few mountain lifts (notably those above Mürren in the Berner Oberland or above Zermat near the Matterhorn), and 25% of the Jungfraujoch above Wengen. Coverage map is here: scroll down to page 2 and zoom in for details: https://www.sbb.ch/content/dam/sbb/de/pdf/freizeit-ferien/ferien-kurztrips-schweiz/internationale-gaeste/sts-geltungsbereich_en.pdf

Both 1st and 2nd class passes also act as Swiss Museum Passes.