Memorial Day can get very crowded. Anytime kids are off school, it's gonna be busy!
The advantages of staying onsite are location (although not as much as WDW), early entry each day it's offered (which is offered right now, but not guaranteed) a private entrance into DCA through the GC (any of the three can use this with room key), GC kids club for all resort guests, and the biggest perk for me is the pools as there's nothing I hate more than a plain square. I love a resort style pool which the DLH and GC both have. PP is okay now because of the slide, but it's definitely my least favorite pool of the three.
The only other perk which cannot be measured monotarily is never having to leave the Disney experience.