Only difference is in experience and training.

A geriatric psychiatrist should have specialized training and knowledge in assessing and treating older clients like your mother.

The geriatric psychiatrist are trained to treat elderly patients, which requires lower drug dosing in patients and being extremely cautious with some medications that should not be used in that populations.

Having said that, most psychiatrist if they manage patients conservatively with minimal increases in dosage will be equally good.

A geriatric psychiatrist deals primarily with elder individuals & their mental health issues. They generally are able to deal with mental health problems associated with dementia.