Half Life 2's presentation cannot be contested as spot-on perfect. While the story tends to be rather short, it could be considered one of the longest shooter game stories in existence. The game always keeps you on your toes fighting the enemies, but it does have one flaw: Occasionally, and I mean occasionally, it has the same issue as Half Life 1 did: You either get confused or bored of it. Half Life 2 broke up the segments much better than the first, and kept it from being so repetitive, but there still are those points in which you want to get it over with.
Half Life 2's multiplayer, in my opinion, beats Halo 2's by so many miles, its incredible. Half Life 2 comes with HL2 Deathmatch (Which I think sucks, but its a lot like Halo 2's multiplayer, so there you go), and Counter-Strike Source. Oh, and Day of Defeat Source, when it comes out. Oh, and Zombie Panic Source, and every single modification for Half Life 2 that ever comes out. Get the idea? Get bored of one mod, switch to another. Go from killing terrorists to hiding out avoiding rampaging zombies. Go from zombies to nazis (I'm feeling the Wolfenstein, baby). Half Life 2 gives you every possible modification you could ever want, and I don't think it could be contested that this is the most expansive game out there, just like the original Half Life.
Now lets quickly focus on Counter Strike Source. A game in itself, Counter Strike was the most played internet action game online, bar none. At any given second, there are about a million STEAM servers running CS, CS:S, HL:DM, HL2:DM, and every single other thing out there. Counter Strike requires pinpoint accuracy, skills to match, and lots and lots of knowledge as to what gun does what in what situation. Sure, its not as complex as Rainbow 6, but there is a hidden complexity to Counter Strike Source, like knowing what map needs what weapon. (Example: In de_aztec, snipers tend to be king, whereas in places like cs_assault, I feel it better to use an AK47 or P90 to blow through the vent on the roof).
Lets face it, though, Half Life 2 is definately not for anyone. First of all, the three packages option tends to scare off many gamers. Second off, it requires a pretty beefy system to run it nicely (Though, it scaled down for my 4 year old Gateway 2000, so it pretty much runs on anything that computes). Second off, players online tend to kick your ass and laugh at you, whereas in Halo 2 everyone's a 12 year old, so it balances out. Lets look at some generic prices:
Halo 2 + XBOX + 3 Months XBOX Live = approx $400
Half Life 2 + Minimum Spec Computer = approx $1000+
Now, that's not to say that both the XBOX and PC can have more games than those, and most of you probably already have a computer of your own. I'd think the PC would be the better bargain because of all the things you can do with it (Hell, you can emulate games you already own on a PC, etc etc), but that's up to you.