Both are fabulous community efforts, and I agree with the sister comment by HN user klaussilveira. Now, they are very different things:

- The Black Parade is a single experience: one campaign with a beginning and end, on the oldschool Thief I foundations. Nothing more, nothing less.

- The Dark Project as of today is more of a “platform”: a modern base engine for creators and players who want a shinier Thief, and who acknowledge that with today’s graphical standards comes extra effort to create a satisfying map/campaign (need bigger assets, less “blunt” architecture, etc). To add to the “platform-ness”: as of today, out-of the box TDP has only a couple built-in missions and no meaty story arc. There are many excellent 3rd-party Fan Missions (maps in Thief lingo, go visit https://www.thiefguild.com/fanmissions/ ) for TDP, but it’s not “a game” the way Black Parade is clearly a game. This is not a judgement call and I had an excellent time with many TDP maps, and community members do discuss expanding the campaign & story... but for now it’s more of a technical foundation to download maps and tinker with, than “a game” :) . You can do some spelunking on the TDP forums if you want more details, the maintainers make no mystery of this.