Killzone 2 is more technically impressive. I personally prefer Killzone's art style too.
Don't get me wrong, Gears is an awsome game (one of my favourites so far this gen, in fact) with some of the very best console graphics to date, but realistically, Killzone 2 just has noticably better graphics.
I mean, just take a look at the environment in this video: http://www.wipido.com/video/QZfyTtiXzKcv












