It depends on the game, so the real question should be "do you prefer single player or multiplayer GAMES", not "content". 'cause, the way you phrased it, what would someone answer if the game in question is... dunno, Quake Live?! "Do you prefer single player content in Quake Live"?! Or in games like Team Fortress 2, DOTA, Pong? There's almost no point in having single player in those - and the same goes the other way too, for strictly single player games. "Would I like multiplayer content in The Secret of Monkey Island", or Heavy Rain? Hell no! :-D
So, to answer that transmutated question, I prefer single player games. I'm more of an introvert and don't like to have to interact with others if I don't have to. Leave me alone, people! Play your own games!
That being said, one of my best gaming experiences of late was multiplayer Portal 2. Having to co-ordinate your movements with accuracy with another individual who could be on the other side of the globe, to save a falling cube from falling to its doom, was a stellar experience.