The Stardust has some kind of show every night (2 showings most nights) such as a comedian, broadway numbers, magic show, or Cirque Du Soleil wannabes. The Spinnaker will have assorted bands and theme dance parties, like a 70s or country night, or a band playing Big Band standards.
Personally, I find the Stardust entertainment rather lame. Sometimes I find the bands entertaining, more for kitsch value than anything. But I'm rather picky and the last couple of cruises didn't bother with any of it.
It just depends on what you enjoy.