I own both of them..
The first halo has a way better story then the second.. When I first played halo 1 and was playing Co-Op (the fun way to play) we were doing the level where the flood are released. As we made our way through the prison and find more and more carnage, but unsure what was the cause we actually had panic fire at just about anything. After you find the recorder in the Marine's helmet and played the footage back, that moment I think is the best part of the game. Once we heard the clanking sound (doors being beaten open after looking at the recording) panic ensued and was the first time I was truely afraid of what was about to happen..
F.E.A.R looks too have a good story and watching gamespys 15mins hands on footage is a real treat.. The guy playing happy goes about his way shooting stuff acting tough like in any shooter.. When the wierd stuff starts happening you noticed him checking cornors more and spinning around a lot. He also panic fires at stuf that isn't "Technically" there a few times.. Good stuff and the Demo comes out Friday =)