My opinion on the matter is that I find Killzone 2 to be all around better developed than any Halo title yet,But Halo is a very well established franchise from the success of the first two games,where as the first two games of Killzone didn't fair as well sadly.
That said,Killzone is probably the best developed FPS from the 7th gen,in terms of how it works anyway,the online is incredibly detailed :) and a lot less crowded than COD if you ask me.I recall many a time playing Killzone 2 online and quickly realising where the name comes from ^_^
But the argument here is fanboyish and irrelevant.TPS shooters are quickly showing that their more challenging and fun to play than FPS if you ask me,but FPS will always be as successful as they are from games by Activision and EA rather than Sony and Microsoft.
For example,Call of Duty,Medal of Honour and Battlefield are all out this year,and the sales from all of them across all platforms will totally eclipse combined sales of Killzone 3 and Halo:Reach.
For any non believers,it's becoming more and more known that you can develop a more detailed game on the Playstation 3 than it's HD rival,therefore I expect Killzone 3 in terms of how well it's developed and how well it plays to trump any FPS.But people won't buy it,because it's far faster than any other FPS which puts COD and multi-plat Halo players off,and COD gets about a bajillion logins a second (btw...thats a slight exgageration but more or less true).
Playstation gaming for 18 years this 2012 
