The letter format is where the conditions of Abigail's accusations become divergent. For example, if the letter is written to the community or to Judges Danforth and Hale, Abigail's accusations would center on how Elizabeth "fits" the profile of those being accused. Abigail might point to how Elizabeth is not a regular at church attendance. It might also point to the poppet that was in her home. Finally, Abigail's letter would focus on how she, herself, has been a victim in this "ordeal," helping to confirm her own accusations as fact more than innuendo and gossip.
If the letter was written to Elizabeth, the tone is different. A letter written to Elizabeth that explores the conditions of her accusations would have to be more honest and more direct. Yet, I think that Abigail would write a letter to Elizabeth that is shrouded in Abigail's steely resolve to not be denied. A letter like this would focus on how Abigail will accomplish what she sets out to do. Given her characterization, she would be smart enough to not reveal the true nature of her accusations, as the letter could be used in court to impugn her. Yet, a letter to Elizabeth would be filled with enough of a message to convey that Abigail has a fixed goal and this will not be taken from her. The audience to whom the letter is written would change the explanation of the conditions of her accusations towards Elizabeth.