I don't think you'll regret it because the lagoon is gorgeous and different from Raro. What I liked most about Aitutaki was actually staying there because it's so small and sparsely populated, but if I'm remembering right it's hard to do a lagoon tour if you're only there a night, though. So if you'd rather not spend 2 nights there a day trip is probably your best bet.
Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime. Mark Twain
The day trip is worth it from what I've heard (DHs cousin has done it a number of times). The lagoon is definately the best part of Aitutaki and I wish we had done more trips out while we were there.
Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime. Mark Twain