Metrics based approach as it indicates from its name depends on the data accimulated over time from the use and development of the software. It is not complete without an expert based approach as they help each others to evaluate best possible test results.
Expert based approach depents on the testing team and their expertese. Having multi-talented and well experienced test experts in the team yield most test products possible. Do not forget that there is no superiority of one approach to another. They are like two feet to move together to walk properly.