Assuming that Arma2 sales are due to Summer Sale and Dayz...I have both already and I have to say Dayz is still so Beta I'm not sure what all the hubbub is about. A cool concept, sure, but it suffers from the same issues the series suffers from.
Too focused on being a "sim" and counter-intuitive controls make it a chore to play. Yes, I'm sure you can remap keys and spend time finding a community so you know the folks you're playing with, hopefully resulting in a less sparsely populated multiplayer session. But, that's a lot of effort when I have a dozen other games I could play.