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 re the fundamental differences between a functional manager and a project manager?, 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?