The value of Orange Box does depend somewhat on if you have played Half Life 2 and maybe Episode 1 yet? If you haven't the Orange Box is a far better value, Theres about 25-30 hours of single player game with HL2 and the two episodes + portal, plus a (supposedly) great multiplayer game. I've only played the single player campaigns of both series, so if you are looking for multiplayer, I can't compare.
Having played both Half Life 2 and Episode 1, and Halo 1 and 2, I'm more excited for the Orange Box even though I have already played a lot of the value. I thought Half Life 2 and Episode 1 were much better FPS's overall. Valve really puts a lot of gameplay variety to break up the action, with the physics puzzles, gravity gun sequences, controlling antlions, etc, combined with great pacing and setpieces. Episode 1 also feels like a coop game, because they did such a convincing job with Alyx.
The Halo gameplay got pretty repetitive for me, the Library level for example was excruciatingly tedious. It's not a bad game, the expansive environments make for some great battles, but when it gets into the corridors the game it loses its steam IMO. I don't think Halo 1 or 2 had enough gameplay variety for my tastes, but for many people, it didn't need any.
I think for a lot of people most of the comparisons on these two games are based on the two stories, for me I think both stories suck. I would maybe even give the edge to Halo because the Arbiter storyline in Halo 2 was interesting. However, Half Life 2 incorporates the story into the game much better IMO, to where you are playing the story, vs. watching it. But I put less weight on the stories than most people.
I am getting both games, Halo 3 is already on its way, and I will be getting The Orange Box for the PC.