The Statendam is HALs oldest ship, and one of its smallest. Expect it to be fairly quiet, especially at night.
The Westerdam is one of the Vista-class ships which has much more onboard in terms of activities. I am sure you will find it more lively and fun.
If you like small & quiet ships then the Statendam will appeal to you. Both have movie theaters, but the Statendams is nicer and less crowded.
The Westerdam will have the Explorer's cafe, Chef's dinner and other upgraded amenities.