One difference in my eyes is that MZB admitted in multiple depositions that she actively aided and abetted her husband, Walter Breen, in the sexual abuse and molestation of children. She was never convicted of any crimes relating to her daughter or other children so on that topic, I remain neutral though leaning heavily towards Moira Greyland being truthful.
For more information, see
DEPOSITION OF MARION ZIMMER BRADLEY - 8/10/98 & DEPOSITION OF MARION ZIMMER BRADLEY, 12/14/98 for the text of the first two depositions.
So far, Neil Gaiman has not admitted to having committed any wrongs.