I'm really fascinated by Labyrinth Lord and Dark Dungeons providing such a similar experience. What is the motivation for producing each one? Is there rivalry? Or is reengineering a BECMI retroclone a hobby unto itself, like rebuilding antique autos?
They're not the same.
Labyrinth Lord is a clone of the 1981 edition of D&D, usually called "B/X" (the Basic Set by Tom Moldvay, and the Expert Set by Dave Cook and Steve Marsh).
Dark Dungeons is a clone of the Rules Cyclopedia and Wrath of the Immortals, specifically with all of the optional rules "switched on." So it has all of the high-level stuff (from the '84 Companion Set, the '85 Masters Set, and the '92 version of the Immortals rules) that Labyrinth Lord does not, and it assumes the use of General Skills and Weapon Mastery (whereas Weapon Mastery was merely optional in both BECMI and the Rules Cyclopedia, and General Skills, which first appeared in the Mystara Gazetteers, didn't appear in BECMI at all).
As retro-clones, it goes without saying that the purpose of these books is to (1) keep old rules "in print" and available to new players, and (2) to "emulate" those rules for the sake of new support materials, like settings and adventures.
Lastly, the plug. Check out
Engines & Empires; it was a setting written for Labyrinth Lord before Dark Dungeons came out, but it's really much closer to the latter game in terms of scale and scope.
