I have done KUL-SIN only three times, second class seat, second class sleeper, as well as 1st Class sleeper (between 1999 and 2006). I have never done SIN-KUL as it's twice as expensive to do SIN-KUL than KUL-SIN. If you want to see the landscape, day trip is better. The Second class sleeper was adequate. The meal in the First sleeper was barely a meal. The best feature was the ensuite shower. I liked being clean and fresh in the morning. The freh feeling doesn't last very long as you are sweaty within minutes of leaving tghe train.

Since you are doing the night trip, a second class seat would not be appealing at all.