The to take advantage of the key hero talent Apocalypse Now of Rider of the Apocalypse you are forced to talent into Frostwyrm’s Fury and Absolute Zero in the frost death knight tree. With Death Bringer, there is no such requirement to make use of it’s key abilities, Reapers Mark and Exterminate. This makes Death Bringer a much more flexible choice whereas with Rider of the Apocalypses you are locked into one specific build.
I think it would be better if “Apocalypse Now” was an independent ability with a 1.5 minute cooldown.