both. In fact, I may actually play more Multiplayer than Single player.
I just find multiplayer dynamics much more interesting than a purely AI world. Not much of a fan of the inbetween. That is, a world like WoW with humans, but a world and story structure pre-built and run by AI. Instead, I like Minecraft, TF2, Savage 2. For single player, the world is much more important than the story (I've played a million and one "story games") to me. I usually need a reason to care about saving the world or saving this person or doing this. In Fallout 3 I didn't really give a shit about my "father", so I spent most of the time doing the quests, which were excellent. In Golden Sun, the end of the world felt like a legitimate threat since you had to travel just about the whole world and see various cultures just to get to that point of revelation, so it felt like "the end of the (virtual) world" was something that actually mattered.