People really need to think about some things:
If mary is not evicted tonight, she'll think she's been proven right, about being the only person in the house who is not fake, and the other housemates will react to this by being afraid to argue with her, because they'll think she's really popular on the outside. There won't be any arguments, or flashpoints with her, because everyone will treat her with kid gloves.
Also, think of the reactions of the person who actually leaves, if it is Craig he will probably cry a bit, and generally enjoy the experience of leaving, he'll ham it up and bitch about everyone with Davina.
But, if Mary leaves first, she will explode, she'll almost certainly tell everyone what she thinks of them before she leaves, then she'll be really angry and vindictive in her interview with Davina, she may even attack some of the live audience, if they boo her.
All in all, the best possible result would be to keep Craig in, and get rid of Mary.