Project Managers work with different skilled workers to meet a specific goal, while Functional Managers manage a specific business unit that may provide services to the project manager.
Based on the answers in the question, What are the fundamental differences between a functional manager and a project manager and is it possible to switch between the two?, the Project Manager may deal with several different business units, such as development, marketing, sales, business development, and many more.
On the other hand, a Functional Manager is responsible for the ongoing success of a specific business unit.
In terms of career growth, which position is more likely to help someone move into an executive-level position?