I think (and am pretty sure) Killzone 2 will sell more. It is in a far more popular genre than Alan Wake, which would be the first advantage it has. It was also far, far more hyped. From what I've seen, Alan Wake is only really being anticipated by people like us on the forum, who don't just play games but actually talk about them a lot too. Killzone 2 is also a far bigger exclusive for the Playstation 3 than Alan Wake is for the 360, as far as I've seen as well. It might not have the same sales, but in terms of name and hype it is equivalent to Halo/Gears of War for 360. Alan Wake will probably end up more equivalent to Demon's Souls (with higher sales though), as it will be far more of a niche game.
That's how I see this "sales battle" turning out at least.