Even though Episode 3 is going to be the stuff we've all been waiting for even talk of, it's not going to come out any time soon or even within the next 6 months (my guess).
L4D2. Because it isn't the same thing over and over again.
Episode 3 is going to be like Episode 2. Nothing new every time you play it. You'll predict when a Combine will appear and shoot you once you play it enough times.
L4D2 changes every time you play it, even when you restart a mission/checkpoint/whatever. You can't predict what will happen when unless you're psychic