I enjoy them both, but have only played a hand full of games total.
Black Plague: shorter play of 1-2 hours. Set up is a bit simpler so travels easier than massive darkness. Mechanics are a tad eadier, which allows bringing a casual gamer into the game easier. Only zombies and necromancers, so once you understand their mechanics game play goes quickly.
Massive Darkness: longer play of 4 plus hours. A lot of minis. After the first travel game, I have decided to only play this at my house as set up and clean up take awhile and I can do it when I want at my own game table. More options in character abilities that should allow for a lot more replayability, but this does make it a bit more complicated for the casual gamer. A lot of different monsters and sometimes their abilities are not clear cut, which can slow down game play. Everyone that has played once to play again even though we got our butts kicked in thae last game.
My Massive Darkness may be a lot more involved as I dumped all the expansions into the game immediately. Whereas Black Plague (and Zombicide) I just play with the core game.