Maven and Forrest primarily focus on different areas, but have some overlap. Forrest is for project documentation, Maven is for standardized project builds. Maven happens to generate some nice webpages for you too if you want.
Forrest is much more powerful for documentation. Maven is (completely) more powerful for building projects. And you don't have to choose one over the other. Use Maven to build a project, use Forrest to build it's documentation. You can even extend Forrest to read Maven's project.xml and generate pages based on that information if you want.