Is there any significant difference between first and second class train travel in Switzerland?
It depends quite a lot on the line. Generally second class has big seats and lots of legroom and big picture windows. First class on SBB main line trains has fewer passengers, bigger seats, often 2+1 across instead of 2+2, even more legroom, business people trying to work, and American tourists who can afford the extra costs.
They both get to the destination at the same time.
On narrow gauge tourist railways the seats in both classes are smaller and sometimes wood.
On scenic routes that have panorama cars, those are usually also available in both classes. A few excursion routes (Chocolate train, Wilhelm Tell Express) are considered 1st-class products and require an upgrade fee if you don't have a 1st class ticket or pass. Whether either with tickets or Swiss Pass, first class costs 50% more.
While first class provides more space per passenger, during rush hour it is often fully occupied and difficult to find seats.
Switzerland does not normally do company cars, but firms do offer discounted annual tickets for public transport and it is tax deductible so many workers splash out for first class. Hence if you are doing a lot of traveling during rush hours it might not be great value.
And don't forget the Swiss trick: if there is a dining car, get a second class ticket, sit in the dining car and order a coffee!
Unfortunately, if the train is crowded, you will be unlikely to find a place to sit in the dining car. Others will have the same idea.