There are differences in the ancient manuscripts and versions for the end of Mark’s Gospel. But what do those differences mean to us?

The argument for the authenticity of Mark 16:9-20: roughly 1000 Greek manuscripts include these verses. Only about three omit them. Even most of the ancient versions and "church fathers" include them. End of story.

The argument against the inclusion of Mark 16:9-20, the two oldest and best manuscripts available for the end of Mark’s Gospel (Sinaitic and Vatican 1209, of the mid-4th century) omit all of these verses, as does the oldest Aramaic manuscript (Syriac, syrs, of the late-4th century), the two oldest Georgian, a hundred Armenian manuscripts, and a Coptic-Sahidic manuscript. Eusebius and Jerome say the verses were omitted in almost all the Greek manuscripts of which they knew.

Some manuscripts and ancient version (translation) manuscripts have a short ending (usually followed by the traditional long ending): "But they reported briefly to Peter and those with him all that they had been told. And after these things Jesus himself sent out through them, from east to west, the sacred and imperishable proclamation of eternal salvation," as given by Bruce M. Metzger, A Textual Commentary on the Greek New Testament, second edition, United Bible Societies, 1994.

Metzger also notes that verses 9-20 have nine (Greek) words and two phrases that are nowhere else used by Mark. And, "The connection between verse 8 and verses 9-20 is so awkward that it is difficult to believe that the evangelist intended the section to be a continuation of the Gospel. Thus, the subject of verse 8 is the women, whereas Jesus is the presumed subject in verse 9; in verse 9 Mary Magdalene is identified even though she has been mentioned only a few lines before (15:47 and 16:1); the other women of verses 1-8 are now forgotten ... In short, all these features indicate that the section was added by someone who knew a form of Mark that ended abruptly with verse 8 and who wished to supply a more appropriate conclusion ... Thus, on the basis of good external evidence and strong internal considerations it appears that the earliest ascertainable form of the Gospel of Mark ended with 16:8."

The United Bible Societies’ critical editions, "The Greek New Testament," omit verses 9-20, with the confidence "that the text is certain." Tischendorf and most other critical editions consider these verses dubious or spurious.