Primrose just softens the cervix where castor oil causes contractions and cervical dilation. I think all women should start taking primrose the moment they start showing any dilation of effacement. You can actually take both together.
I would honestly say don't do castor oil. The risk is just too much for making baby sick if he/she has a bowel movement in the womb and inhakes it. If you can stomachs tea I hear raspberry leaf tea is a great starter, along with nipple stimulation and even sex.
Castor oil for effectiveness and primrose for healthiness.
Primose is more healthier because its natural castor oil works faster got to give it a few hours tho
Would not use either of them you'll just get super bad intestinal cramp and the super poops