I think Aitutaki is definitely worth the trip. It's simply gorgeous and a special spot. And the plane trip has some amazing views as well.

As for spending 3 nights there, Aitutaki is a very quiet place, it's really all about the natural beauty. Raro has a lot more options for eating, drinking, etc. Some might get bored in Aitutaki, especially if they don't want to snorkel/swim. Although, if you are staying in the resort on Aitutaki it might not be much different than a resort in Raro.

Definitely at least do at least a day trip to Aitutaki, but whether you spend all your time there or not depends on what you are after.