I'll reply having actually read that you already own TF2 and HL2.
You're looking at different games here, so it depends. If you want single player, I agree that the orange box is probably your best bet, as it nets you Portal and Episode 1 and 2 (too bad you missed the last 2 50% of everything half life). If you liked Half Life, more Half Life is good, and I very much enjoyed Portal's puzzles. It gives you a good sense of accomplishment when you launch yourself over some gap or obstacle. Yes it was short, but it's not a full-priced game, either.
For multiplayer, go L4D2, no real point in picking up the first one really, as the second is the "new and improved" and will probably be maintained longer than the first.
I personally am skipping L4D2 because I rarely have a solid hour to play through a campaign/versus, so single player games and TF2 are my mainstays.
If you were to hold a gun to my head...I'd probably ask you why you were holding a gun to my head and make you answer the question: "Single player or multiplayer?" That answer is your answer.