I am going in a week and we have reservations at both. Going with my wife and her family.
It really depends on what you are looking for! The Golden Nugget just got a sweet remodel, and is looking hot for an old lady on the stip! http://www.bestofvegas.com/hotels/Golden-Nugget-Las-Vegas/ it is classic old Vegas, but because Downtown is full of nostalgic older guests you do get a more mature crowd.
Monte Carlo is great, and a lot of fun and has a much younger crowd http://www.bestofvegas.com/hotels/Monte-Carlo-Hotel-and-Casino/ but it lacks that Old Vegas charm.
Anyways I hope you have a lot of fun!