I would go with L4D2. I love the Left 4 Dead games, and for 20 bucks you get a hell of a lot of game-time. They're pretty much infinitely replayable. I grabbed the L4D pack a week before Halloween for $15! Either way, I'd save your $20 until around Black Friday because Steam will probably have a ridiculous sale and you might find L4D2 or another game that you really want for around the same price.