I don't hang out in First unless somebody else is paying - but I get Jazz+Travels's comments about social distancing - but my memory, having been on the Zentralbahn's Interlaken - Luzern trains quite a few times, is that the First class area is at the front and back of the train, and the Second is in the middle. The First class compartments are quite small, and have a door so there will actually be less air flow. One other consideration is that because you are at the ends, there is the wall of the driver cab, so your vision to the front or back can be restricted, and I think I remember that because the First Class is over the wheels it is up higher so the panoramic windows are actually shorter than in Second.
The views are good both sides, and the train is narrow because of the narrow gauge cogwheel train track, but the best views are sitting on the right leaving Luzern. One of the most pretty rides in Switzerland IMHO. When the train changes ends in Meiringen front becomes back and v.v. and you are still on the better side as you go along Lake Brienz - Brienzersee.
See how many lakes and waterfalls you can count. Some waterfalls will be the same one from different angles, especially that one at Giessbach.