I would avoid a grad night whenever possible. Not just higher crowds, but large groups of high school kids walking around, blocking pathways, cutting lines (sending 1-2 people in the line and then 5-6 others walking through to catch up), and single rider lines are just super blocked. I think that will more than make up for slightly lower AP crowds.
But at least your crowds for the first several days will be pretty low, so if Saturday is more convenient for you, you can always try, just accept it may be annoying. At least more of the grad night kids will be in DCA by the time of parade and fireworks.