Everybody building a web application with a modern framework, is already probably using an ORM (Object-Relational Mapping). Most frameworks include one out of the box. But digging deeper into the subject, ORMs do vary from each other, and some cases, very significantly.
The two approaches seem very similar. The difference is described in a multitude of articles online. I particularly liked this one. In essence, Active Record is a better choice for simpler, CRUD-based applications. Data Mapper, on the other hand, is better for domain-specific applications, as it provides another level of abstraction between the domain objects and the persistence layer.
The CakePHP ORM borrows ideas and concepts from both ActiveRecord and Datamapper patterns. It aims to create a hybrid implementation that combines aspects of both patterns to create a fast, simple to use ORM.
It looks like I need to do some learning and dig deeper into the subject. Pointers are welcome.