I still believe the PC version as sold more copies via steam. We will never know unless valve releases some figures.
But anyway L4D is just a really fun game, someone stated that it only has 4 maps. It has only 4 maps but it plays diffence everytime you play. For the best experience you gota play with other people, I found single player just too boring. But I have cyper space friends on Steam who I play with all the time and the fun never ends.
I only play co-op on expert now.
PC gaming rules.....







