Recently, I have been studying I and II Peter with a group at North Metro Church in Atlanta. When it came time to look closely at II Peter, I insisted that we also look at the short book of Jude.
There is a definite LITERARY relationship between these books. Clearly, one author had a copy of the written text of the other and frequently quoted and reshaped the source text. This is much more than simply an appeal to the same oral tradition. The vocabulary, syntax, and even sentence structure betray a literary dependence of one document on the other.
While I will not attempt to argue "who quoted who" in the II Peter-Jude interdependence, I would like to take a moment and offer several observations about the phenomena of literary dependence in New Testament texts that apply beyond the II Peter-Jude context--especially in the literary interdependence of the synoptic Gospels.
When one biblical writer uses the written text of another biblical writer, there is a tendency to (1) soften, (2) shorten, and (3) embellish the text of the source document.
Shorten - The quoting writer will often reshape the original text to make it more pithy--that is, easier to tell and remember by the removal and/or replacement of (a) technical language and (b) local detail.
Soften - The quoting author will often remove or smooth out controversial ideas or words, especially if they conflict with the agenda of the new writer. This is most clear when the quoting author takes special care to remove embarrassing and/or easily misunderstood content from the original text.
Embellish - The quoting author will often add detail and/or language to the original text that will further his own theological message. (While this may seem to contradict the principle of "shortening" the original text, this is an entirely different matter. In "shortening" the text, the new author removes content that is superfluous to his theological agenda. In "embellishing" the text, the new author adds new content not present in the original text for the express purpose of furthering his own theological agenda.)
These three tendencies are everywhere apparent in Matthew's and Luke's handling of the Markan (triple tradition) material. Such tendencies are not as clear in their supposed "manipulation" of the so-called "Q' (double tradition) materials.
This speaks loudly for Markan priority and much less for the possible existence of the theoretical "Q."