Travel / Train from Warsaw to Vilnius/kaunas [8]

Hello,

Can anyone tell me the difference between first and second class on this train? Is it worth the additional cost? Sometimes it doesn't seem an better on Polish trains to upgrade and the only difference is the color of the carpet in the 'box'

Any information on the seat style? Meaning boxes or individual airplane style seats?

Have a bad back so looking for as comfortable of an option as I can find.

Thank you