Is there a significant difference? My only train-class experience was in Italy, where I found the 2nd-class cars perfectly acceptable. I'll be traveling on a weekday morning in late September, if that makes any difference. I'm buying two tickets, so the 2nd-class savings of 68 euro is substantial--although I'd be willing to pay extra if it were a matter of having to stand instead of getting a seat.
I'd intended to spring for the Thalys train but I didn't realize that the prices rise as the departure date approaches, and now it costs more than twice as much as the IC train. (I'd be willing to spend it if there weren't an alternative.) I don't mind that the ride is 2 hours instead of 1, but I am a little nervous about changing trains in Rotterdam--does 17 minutes sound like enough time to transfer?