The best of both worlds approach views project management as both an art and a science. Being too loose with the project management process can lead to a scope that keeps growing out of control. Likewise, being too stringent with adhering solely to the project management methodology could lead to missing important moments where a creative approach could propel the project forward. So what's a project manager to do?
Since project management is mostly about achieving a desired effect through productive measures, the "art" part of project management comes into play when you are dealing with human behavior and interpersonal relationships. Getting Bill to do what needs doing is the art of project management. Creating the project schedule, dealing with quality management, and estimating costs are best left up to a best-practices approach.
Finally, viewing project management as both an art and a science can be beneficial when your project comes to a standstill. Viewing project management as an art will help you to see creative solutions to problems. Viewing project management as a science will have you create that risk management plan before you even begin to schedule your project - with appropriate and creative solutions to potential risks.