Not sure I understand the reliance on metacritic to decide what is good and what isn't. Review scores at the beginning of a generation are always higher than at the end, even if the games themselves have improved. Just look at Halo.
As far as one game selling more than another, it's apples to oranges when you're dealing with different genres. Perhaps more people simply like hack n' slash than third-person action games, or perhaps one was marketed much better.
Personally, I like Killzone. It's one of the few sci-fi console games with a degree of complexity and originality to it. Most of the rest are straight from decade's old pulp novels. Killzone reminds me a lot of Dune, actually, with the politics, power struggles, and moral uncertainty. There is more to it than meets the eye, which is apparent by the way people still make threads to discuss the story.
