In terms of action, the two franchises are equally fast paced and gory, however, Splatterhouse has always been a more mindless free-for-all type game whereas Darksiders involves more thinking especially with some of the game's more intricate puzzles. Overall, Splatterhouse is an action game at heart whereas Darksiders is both action and adventure. And it's in that one simple word-adventure-that key differences are highlighted.
Haven't heard of the comparison myself. My initial reaction to this question though was "is this some clever thread making fun of how all this game seems to get is comparisons and not questions about the game itself?"