I think the needs of the charity or nonprofit organization determine whether volunteering or donations would most greatly benefit their cause.
Many organizations have sufficient revenue to support their operations but are in desperate need of additional man power to help meet the needs of the clients they serve. While others may have an ample rotation of volunteers; but struggle financially due to budget cuts and restraints.
Seasons also affect an organization's need for volunteers or donations depending on their cause. Many have great demands during the holidays, summer, and winter months.
I suggest contacting the organization that you are interested in supporting to determine what their greatest need is.