Buy 'em both!
They are both good books, each in its own way. If you want 'pure theory', then Silman/Donaldson is the choice. If you want some discussion of plans and a lot of illustrative games, then go for Denmark.
In fact Hansen/Nielsen is of the now familiar 'complete games'-type of book; S/D is the 'old' style, which some people call the 'telephone book' style. My impression is that H/N is very instructive, while S/D are more reliable on analytical grounds. I recall that one of H/N's notes in the Gurgenidze, given as the way to go for Black, just loses a knight...
By the way, there are two good repertoire books, based on the Accelerated dragon.
"Winning with the Sicilian Defense" (1998) by Silman is an extract of S/D, but also covers the Anti-Sicilians.
The new book by Alburt/Dzindzi/Perelsteyn(?!), "Chess openings for Black - explained" is also quite nice; but it covers a complete repertoire for Black: Sicilian AD, Nimzo-Indian, Bogo-Indian, Symmetrical English. It' very readable, but to me it seems a bit superficial in certain lines.
However, if you want an overview of the whole Accelerated/Hyperaccelerated Dragon, then it's either Silman/Donaldson or Hansen/Nielsen.
I also like Levy's book from the 70ies, but it's of course totally outdated.