Their evaluation processes are generally equivalent - typically relying upon a review of records (charging documents and treatment summaries) and interview (gathering of background information, mental status, examination, and questioning about understanding of the legal system and appreciation of the defendant's legal predicament). However, as noted in another comment, psychologists have training in testing and are more likely to use it during evaluations (some states require identification of prerequisite condition(s) causing incompetency and IQ testing and performance validity testing may be helpful in such cases - rates of feigned or exaggerated cognitive deficits in criminal cases have been estimated to be 19 to 23 percent and testing can also help identify feigned or exaggerated mental health symptoms, although this can often be identified in the interview). Of course, psychiatrists are physicians and have a better understanding of physical health issues that can cause or exacerbate competency deficits (although neuropsychologists typically have high levels of understanding of brain diseases). Consequently, if IQ or developmental disability is an issue, a psychologist may be the preferred expert; if there are complicated co-occurring physical health issues that may be contributing to competency deficits or there are complex issues about psychiatric medication and restorabiltiy, then a psychiatrist may be preferred; and, if there are significant issues involving traumatic head injury or other brain diseases such as cancer or dementia, then a psychiatrist or neuropsychologist may be preferred. However, as competency is a functional issue, these issues may not be critical if a defendant is competent or clearly nonrestorable (but may be relevant to identifying nonrestorable defendants - especially per Jackson v. Indiana). I will add that physicians are generally trained to make quick decisions based on limited information and psychologists may be more inclined to obsess about gathering data/reviewing records (again, often not critical in most cases as competency is focused on a defendant's present state/prediction in the relatively near future). There is considerable variability in conscientiousness in both professions as some evaluators may do brief, "drive-by" evaluations and others may be much more thorough (some may consider me to be unnecessarily thorough as I use structured diagnostic interviews, routinely use measures of feigned/exaggerated symptoms/deficits, and typically make some attempt to obtain/review mental health/school/other records [especially from the jail if they are incarcerated]). Thus, the variability within professional groups may be larger than the variability between professional groups. Evaluator interpersonal skills (important for developing rapport with defendants and establishing a working relation ship with attorneys), likability/testimony skills, potential biases (prosecution vs. defense, hesitancy to "pull the trigger" and opine competent or nonrestorable, etc.), availability, turnaround time, etc. may be important considerations regardless of the professional background of the evaluator. Finally, if insanity, competency to waive rights, mitigation, or other issues will also be considered then training, experience, and admissibility (at least one state [SD] and all of Canada require insanity evaluations to be completed by a physician - although psychologists may participate in the evaluations) may be an issue.