Killzone 3: ~64%
Call of Duty: ~8%
Both: ~18%
Neither: ~10%
If you compare this results to the probable sales of both games, you will reach the conclusion that the average PS3 forumite is not representative of the average PS3 user. Take this into consideration when you analyze sales, and games don't sell the way you'd like.
http://vgchartz.com/swlaunch.php?reg1=All&game1=Killzone+2+-+PS3[12253]®2=All&game2=Call+of+Duty%3A+World+at+War+-+PS3[23993]®3=All&game3=&weeks=65