Back in 2001, Marvel published a trade paperback collection titled "The Fall of the Mutants" which collected the story arc wherein Apocalypse first challenged the mutant superhero teams (X-Men, X-Factor and New Mutants) in pursuit of his philosophy combining Darwinism and Genghis Khan (mutants will grow stronger and more fit to survive if you keep trying to kill them). This was the story where Warren Worthington III became the blue skinned, artificially winged avatar of death. This one-volume collection reprinted Uncanny X-Men #224-226, X-Factor #24-26 and New Mutants #59-61.
Then, in 2013, Marvel issued a two-volume trade paperback collection that collected more of this "Fall of the Mutants" story arc, reprinting all the stories from the earlier book and more. Volume 2, the book currently under review, provides only the X-Factor stories but more of them: issues #18-26, as well as the crossover stories from other titles such as Incredible Hulk (#336-337), Power Pack (#35), Daredevil (#252), Captain America (#339) and Fantastic Four (#312), where those other, non-mutant, characters had to deal with the fallout of Apocalypse's attack on New York and, in the case of Cap, on the Midwest farm belt. (The Hulk story really isn't part of the Fall of the Mutants arc, but has X-Factor during the same time frame investigating a possible mutant sighting that turns out to be the Gray Hulk rampaging through Sparta, Illinois, the town where comic books were printed at the time.)
Thus, you have two versions of this story arc, the original one-volume compilation and the more recent two-volume expanded version. Both contain the core story, but the two-volume set gives you a much more complete and comprehensive account. The main value of this second volume is the crossover stories, none of which are essential to the main plot, but several of which are excellent stories. Of particular note are the Power Pack story, written by Louise Simonson, where the kids try to save their mom who is stranded in the subway during Apocalypse's attack, and the Daredevil story which was written during Ann Nocenti's excellent run on that title and exemplifies why it was so excellent.
If you want everything, you should get the two-volume set. But since I already owned the one-volume version, I found it enough to just buy the second book in the two-book version, since I was only getting a repeat of three X-Factor stories and the rest of this 400-page book was "new." But you can look at all three product pages and decide for yourself.