That may just be a mistake on your booking papers. The Zaandam usually sails to Alaska, Hawaii and Mexico. Zuiderdam being the bigger ship usually sales to Europe. I have never sailed the Zuiderdam but I hear she is wonderful ship. I have been on the Zaandam. And I can tell you that you will enjoy your cruise on HAL whichever ship you are on.
I think you had better call your TA immediately, as the Zaandam is sailing the Panama Canal route in April/May and then it does the Australia/New Zealand South Pacific route in May.
Zuiderdam will be going to Italy the end of March for 3 weeks in dry dock and then she will start sailing in Europe. It looks like the May 8th cruise is from Venice to Barcelona.
We just got off the Zuiderdam's Xmas cruise, and had a good time. There is a lot of refurbishments required, like replacing all the chairs on the balconies and around the pools, but this will all be done when she is in drydock.
Someone has made a mistake in your papers, so I would check this out ASAP.
Zuiderdaam is the newer and bigger ship of the two. It was built for Caribbean cruising and is considered to be the most "Carnival - like" ship in the HAL fleet. A little outlandish in decor, but it makes up for it in amenities. It is more active than the smaller Zaandam - better nightlife, bigger shows ans casino, etc.