The issue, to me, is not the number of days. It would be hard to think of two more different ships than the Volendam and the Noordam. The latter is the oldest ship in HAL's fleet, by quite a margin. She's a lovely classic ship and if that's your taste (and she has a very loyal following) she's just fine. Volendam, on the other hand is one of HAL's newest... veranda cabins, large picture windows in her regular cabins as opposed to port holes and all the amenities associated with recent cruise ship design.
She is also significantly larger than the Noordam, for what that's worth.