I will answer your question in 5 years when I have added 5 more cruises to my list.
Currently I am not qualified to answer having only taken 2 cruises with both on the same line.
Although the Zuiderdam was better than the Amsterdam for all-round enjoyment.
Finally getting back on a Dam ship, Nieuw Amsterdam Nov'11 (My Mom's 80'th B'day cruise)
Navigator of the Seas 2010
Serenade of the Seas 2009, 2011