A psychologist spends more time on the emotional aspects of having ADHD and can treat co-existing conditions such as mood disorders and anxiety. Coaches help clients develop skills and strategies to overcome executive function deficits such as planning, organizing, and time management. Coaching provides more accountability, so a coach may offer more frequent (possibly daily) contact between sessions to help keep clients on track.
Coaches can — and often do — work on emotional challenges to some extent, and psychologists work on life skills. The difference is the degree to which each need is present. If the primary challenge is managing or understanding emotions, particularly those that have been present for years, a psychologist would likely be the appropriate option.
Likewise, if the primary goals are getting organized, managing time, and creating healthy routines, coaching might be the better approach. It is not uncommon for a person to be working with a therapist and a coach at the same time.
In terms of certification, psychologists must be licensed by their state in order to practice, but certification is optional for coaches. A credential is a good indication that the coach has the appropriate level of training and experience to be effective.
Besides certification, you should always interview a prospective coach and make your own decision. Ask questions about her experience and training, and always make sure you feel comfortable with her.