I bought both.
Got Orange Box on release day and am just about to finish Episode 2. It's been a LONG experience with the single player. Seriously, well worth the cash on that end. Played some Portal. Haven't touched TF2 yet ... not sure I'll get to it.
Because of COD4. While the single player is decidedly shorter than the Orange Box's, it is definitely "up there" in terms of adrenaline, gameplay, sound and graphic design. The multiplayer ... I don't enjoy as much as Halo 3's multiplayer (Halo 3 single player left MUCH to be desired, esp. in terms of storyline), but it COD4's multiplayer IS very fun.
Personally, I prefer COD4. But someone suggested renting them first, and that is probably the way to go...
...unless you do what I do, which is buy one, play it, sell it on eBay (or even trade it here), and then get the other one you want. I tend to "lose" $5-10 this way (buy a new game for about $53, sell it for $45 or so, use that money towards the next game). Otherwise, it's all "buy, buy, buy."
Really, though, you can't make a bad decision between the two.