I think ADHD might be better suited to being a regular beat cop (at least combined type). Police have to deal with overloading situations like people yelling and trying to attack each other, and ADHDers tend to be more sensation-seeking rather than hypersensitive. Plus, police are expected to be physically fit, and exercise has been shown to help people with ADHD focus better.
Autistic people might be better suited to crime scene analysis, because it's a very detail-oriented position and not very social.
Of course, both of these conditions vary greatly among individuals, so there will be exceptions to this.