Amongst the many angry reactions to "One More Day" were complaints that it was amoral for Peter Parker and Mary Jane to give up their marriage to save Aunt May's life. Some readers argued that they would never have made such a sacrifice, although this assumes that two fictional characters would respond to an outlandish situation the same way as a particular reader.
While you could argue that it would have been characteristic of Spider‑Man to reject a deal with a shady figure like Mephisto, this seemed to be a situation in which either decision would have been appropriate. Joe Quesada argued that Spider‑Man would have probably made the deal to save the life of a random stranger. He believes that it was important that Mephisto sought Spider‑Man out, instead of the opposite.
What do you think Peter would do if Mephisto came to him and said, “Do you see that woman standing there on the street corner? I know you don’t know her, but I’m going to kill her. Or, perhaps “She’s dying of cancer and I can save her.” However, if you give me your marriage, it’ll save her life.”
How would you feel, how would fandom feel if Peter said, “Nope, sorry, go ahead and let her die.”
The outcome might be in more doubt if MJ's fate wasn't quite as bad as May's. For example, if a bullet had paralyzed MJ. Or if she had faced a life-threatening surgery in her immediate future. The latter example doesn't quite work with Mephisto, as it invites too much second-guessing. Why would he make the offer unless he knew she was going to live?
It probably wouldn't be in character for MJ to agree to the deal in either situation. And if it's just Peter who gets to decide, the "One More Day" angle changes completely.