I'm probably going to get flamed for death for saying this, but the story in the Half-life series is very similar, if not worse than the story in the Halo series. The two differ in the fine detailing but are actually rather similar, it's the way the story is told (i.e barely told) in Half Life that makes it so revered. In my opinion, there is little story in Half-Life and the story in Half-life 2 is so cryptic that I had to go on wikipedia. At least in Halo you know whats going down.
But VS threads are stupid and both games are very good (I'm actually playing Half-life 2 now) and shouldn't be fought over.