Have sailed both...just did 14 days on Zaandam. Nothing wrong with either. I would however, take Koningsdam, as it has more entertainment and dining venues.
I take understaffing comments with a truckload of salt. People blame everything on understaffing. Conversely, after 14 days on Zaandam that was totally jammed to the gills (Eclipse cruise), I never felt there was any shortage of staff, in any way, at any time.