It all depends on what you are doing! First Class is often not available in Eastern Europe. Once you are beyond Wien most mainline trains have first.  If you plan to use international TGV trains the supplement is higher in first than second. Second class is absolutely fine for almost any journey. The only country with decent perks for first class is England where some companies give you free food and drink!  I always travel first class because I prefer a less crowded environment and spend most of my time in Austria Switzerland and Germany where there are no supplements. On trains without compulsory reservations or supplements it is easier to get four seats together in first. As we say in England it’s horses for courses.

If you can afford get first class. The difference may be small but well worth it in many express and high speed trains, especially to those of us of mature years.

However if you are off backpacking and hostelling save your money for treats like a decent meal.

If you are travelling for the full 30 days the cost of any pass will be minor compared to the rest of your spend - go first.