if you're referring to this hotel
http://www.hotelmaas.nl/
I stayed there last year, was pretty good
I had a room with a canal view, would say it's definately worth the small extra
It was my first time to the Dam so didn't know how far exactly I'd have to walk to get to the RLD, it took around 30 - 45 mins to get there, it wasn't a major problem but next time I think I will go somewhere closer.