If the NF in Mono was your first choice, I'd go for that! It's the perfect bag to throw your stuff into, like you mentioned! Good luck with your decison, hope you're able to get your bag soon!
Guess it depends on what you're using it for...Are you looking for a tote? Then I'd go with the NF. The Monty is a nice bag, but the PM won't hold very much. GLD!!
out of the two, the NF is the best. I'm personally not fond of the Monty. I also have the Damier NF and have never had any issues with the straps. It's the perfect tote/knock around bag and always looks great!