Occupation in 1st and 2nd class varies. It is not necessarily so that 1st class is less busy but that can certainly the case, depending on the trains, time of day, day of the week etc.

The only general thing one can say is that with a first class pass, you have a slightly higher chance of getting a reservation in busy trains. Remember that with a 1st class pass, you can also travel in 2nd class.

@Family Trip 

as rvdborgt said You have a slightlyhigher chance to get a reservation as you can take travel in both classes :)

one example for Occupation
The Eurocity from Berlin to Prague often have only one 1st Class Coach but 5 2nd Class coaches. With standard German tickets you get often in 1st Class the seat for free means that especially on Friday the 1st Class coach is complettly full but there are still free seats in the 2nd Class.  How ever a reservation on this route is not compulsory but i recommend one on that route.


 

I’ve had both 1st class and 2nd class Interrail tickets. It’s true you stand a better chance of getting a seat on some trains because the system will offer you a 2nd class ticket if there’s nothing available in 1st class. 

On the other hand, I’ve been on the Thalys 4 times between Amsterdam and Paris with a 1st class Interrail ticket and never yet managed to get a 1st class reservation. :-( 

On the Eurostar from Amsterdam to London I had the opposite experience though - the 1st class reservation is “Standard Premier” class and I had a whole carriage to myself. Something about Interrail reservations being in a different section of the train than normal reservations. I was working on the train, and when the staff came to deliver my meal, they laid it out at one of the other tables so I didn’t have to close my laptop. 

1st class was wonderful on the Great Western train in the UK from Reading to Cornwall. I hadn’t made a reservation (I read a thread somewhere saying there are always a few seats left unreserved for people who buy a ticket at the last minute) so was a bit worried about getting a seat. Imagine my surprise when, a couple of minutes after I sat down, a well-dressed man appeared with a trolley and asked me what I’d like for breakfast - all part of the service. They reappeared with coffee, biscuits, chocolates and sandwiches several times!

For me 1st class is mainly worth the difference if you’re travelling for work as the seats on most trains have power sockets (although lots of them do in 2nd class these days too), but if you’re travelling for 3 months it might be a wonderful luxury to have.

@SamCritch don´t forget the Panoramic Coaches of Swiss Rails on few Eurocity´s across Germany (EC8/EC9) and Austria (EC163/EC164) Few pictures of the Swiss Panoramic cars. on EC8/9/163/164 you need just a 1st class pass

Thank You for the answers!