Different mechanisms are believed to be at work in the two diseases

A group of Norvegian researchers of Stavanger University submitted 488 patients with Parkinson's disease and dementia and 488 patients with Alzheimer's disease and dementia to a neuropsychological work-up using well known rating scales (MiniMental State Examination MMSE and the cognitive subscale of the Alzheimer's disease assessment scale ADAS-Cog.
They established that orientation is more impaired in patients with Alzheimer's disease, whereas attention is more impaired in patients with Parkinson's disease; memory was impaired in both groups, but the severity of memory loss was greater in patients with Alzheimer's disease.

The accuracy of the diagnosis based exclusively on neuropsychological findings was  74,7%

Source: Bronnick et al J neurol Neurosurg Psychiatr 2007; 78: 1064-1068