I always wondered if I was the only one who felt that way about Halo.
Half-Life definitely made the bigger impact. It was the first shooter that was more than straight forward action, and not only put you in the role of the character, but gave you a compelling story, and puzzles that required more than simply retreiving the blue key to open the blue door. Half-Life sparked a wave of immitators, and it's effect on the genre is still present to this day. No FPS since has had anywhere near the same impact on the genre.