Experienced before 1st class train Warsaw/Prague, it was a compartment, albeit the whole compartment feels like reserved for me - because I was alone in it.
Then another 1st class Warsaw/Berlin, where there are still 4 seats per row (2/2).
Then another 1st class Warsaw/Berlin, where there are 3 seats per row (1/2)
I'd normally check the seat allocation first of the train for first class before choosing it. Because if its the same as in 2nd, then there's no point (at least for me)
But if you want peace, definitely 1st class - less crowded