Margaret is just more flexible. Not only is Megan overdone, she might prefer the nns Maggie, Meg, Greta, Daisy, Peggy, or whatever. I agree that Megan is cute to some degree, but I'd rather see a Margaret :)
Well,In my case, though I doubt I'd ever use it, I couldn't use the name Margaret. I could, however, use Megan or Daisy or something. And couldn't meg be a nn for Megan?