The selection of plant part depends either on ehnobotincal knowledge regarding that plant or reported phytochemicals. As data regarding brain activities of these plants is scarce, i recommend you to screen all the available extracts of different solvents for in-vitro activities and select the best one. If not possible I further recommend you to start with aqueous extract as many novel activities have been reported with this extract.
To find the best extract first you have to look on the chemistry of both these material to find out which specific component is effective for brain. Then search the literature regarding that component and search for the best extraction solvent for that specific component and use that solvent for maximum extraction of target component.