I would advise reading through the reviews for Maasdam on this site (cruisereviews.com). Zuiderdam is definitely newer - if I am correct, she isn't even out yet, but perhaps someone out there remembers the date she is due to become officially part of the HAL fleet? Anyway, Zuiderdam is bigger (1800+ passengers to Maasdam's 1300) and will have an alternate restaurant (besides the buffet) and alternate theater - Maasdam has neither one. I haven't been on Maasdam yet - going in 2 weeks!
Prices for the new ships are always higher. Something to keep in mind is if Zuiderdam isn't completed yet, the possibility of delays in her completion could lead to cancellation or substitution of your cruise. [img]/infopop/emoticons/icon_confused.gif[/img]