Ladies and gentleman.... I can ease your pain..
this is what's happening:
Nov 4/5
The Best of 1990-2000 limited edition is released.
Limited edition includes a Bsides disc, and a bonus DVD
the bonus DVD is called the "History Mix" and includes a few live performances are videos.
December
The Best of 1990-2000 DVD is released featuring promotional videos for the songs on the Best of 1990-2000. This is not the History Mix DVD. This has much more.
What may be causing confusion is the question of the History Mix's content. As to what this is, I dont think anyone knows yet. (unless it's like the video montage they're showing on U2.com right now)
Moreover, it has not been made clear as whether or not the History mix DVD is actually a third seperate disc, or whether dvd content has been put on the Bsides disc. I'm thinking it's the former and not the latter. Although I'm not sure how it will be packaged if this is the case... maybe in a seperate cardboard slip?
anyhow, hope this helps!