IMO HL2 gives alot more atomosphere than Halo does (though I haven't played Halo 3), granted you don't have one-button grenade, most times you won't need it. Also, HL2 introduced us to gameplay that lets you interract with most objects in the game once you get the Gravity Gun (which kicks ass by the way).
This game is well worth the $$, IMO HL2 is worth the $60, but as others have pointed out you get TF2 as well as Portal.
As noted in my bitch thread about the PS3 games getting pushed back, HL2 for the PS3 does not support mouse/keyboard which means (unfortunately) I'll be getting it for the 360.