Overview
The two bays are geologically one landscape, divided only by an administrative line and by the licensing that sends most of the fleet into Ha Long. Bai Tu Long has fewer developed caves and more fishing villages, and its national park covers forested islands with beaches you can land on. The trade-off is that there is less to see in the sense of named attractions, and more in the sense of empty water.
Highlights
Choose a two-night cruise rather than one; the second day is what you came for and the first is mostly transit. Cruises leave from Hon Gai and Tuan Chau, three hours from Hanoi. Kayaking is the way to use the time, and the better operators include it.