Some scholars believe that the ending of Mark is unoriginal due to it appearing to have a different style and the fact that it's missing from some key manuscripts. At the same time, several early Church fathers are believed to have quoted from these passages. What are the primary arguments for and against its authenticity (along with sources, please)?
My understanding is that a strong majority of scholars (including conservative scholars) take the position that the long ending of Mark was not in the original and was not written by the same author as the rest of the text, but nonetheless was added very early on (probably in the early 2nd century). However, the evidence is not as overwhelming as for the Comma Johanneum or the Pericope Adulterae, in part because the long ending of Mark is significantly older than the those two.
The main arguments are as follows. Certainly people dispute some of these arguments, but on the whole each of them is a strong argument, and taking several together gives an even stronger argument.
The textual evidence, which covers the first three arguments, as found in Nestle-Aland is summarized ably at the end of the Wikipedia article under "Summary of manuscript evidence." At any rate all of these are largely undisputed facts, though there are some interesting features of that part of Mark in Sinaiticus and Vaticanus which are worth noting (explained thoroughly with images, though also with a bit of an agenda, here).
A quick summary of the argument from style and vocabulary is given by Bruce Metzger in A Textual Commentary on the Greek New Testament excerpted online here. A much more in depth examination is given in a paper of Travis Williams.
For the last point, you want to first notice many similarities between the long ending and Luke/Acts (as well as a small overlap with the great commission in Matthew), and then you have the trickier point of arguing that it's the long ending taking from Luke and not vice-versa. Wikipedia lists the overlaps (I haven't found a good scholarly resource for this point, though of course you can compare the passages yourself). I haven't found a good reference for arguing in which direction the borrowing is going, but if you want to argue the other way you'd need to explain why Matthew drops almost all of it (despite containing 94% of Mark) and why Luke substantially rearranges it (despite usually following Mark's order reasonably well).
It's worth noting that unlike with say the Comma Johanneum, I don't think it's been conclusively proved that the long ending of Mark is not original. Assuming it was added, it was added earlier than any copies of the text that we still have! All sorts of things are unlikely but possible. Nonetheless it seems the evidence is pretty solidly on the side of inauthenticity.