Trophies add a lot more gameplay.
A must buy if you liked RDR.The story isn't that long but worth the play.
If you do the trophies/ achievements there is hours of game play in single player and online game modes. I've probably played this longer than the original because of the ammount of additional trophies they added.
I would say theres a lot more than the stated hours of gameplay, certainly worth the money.