A developmental pediatrician is a regular pediatrician...an MD who specializes in developmental disorders. Neuropsychologist is a PhD who specializes in how the brain is connected to behavior, learning style, developmental milestones etc. Big oversimplification but is that what you were wondering?
to decide which is better, it depends on your goal and their practice.
Internet info:
neuropsychologist: Clinical neuropsychology is the application of neuropsychological knowledge to the assessment (see neuropsychological test and neuropsychological assessment), management, and rehabilitation of people who have suffered illness or injury (particularly to the brain) which has caused neurocognitive problems. In particular they bring a psychological viewpoint to treatment, to understand how such illness and injury may affect and be affected by psychological factors. They also can offer an opinion as to whether a person is demonstrating difficulties due to brain pathology or as a consequence of an emotional or another (potentially) reversible cause. Clinical neuropsychologists often work in hospital settings in an interdisciplinary medical team; others work in private practice and may provide expert input into medico-legal proceedings.
Dev. Pediatrician:
If your child has a developmental, learning, or behavioral problem, a developmental-behavioral pediatrician has the training and expertise to evaluate and care for your child. Developmental-behavioral pediatricians possess training and experience to consider, in their assessments and treatments, the medical and psychosocial aspects of children's and adolescent's developmental and behavioral problems.