You know, because that trailer moved so fast I really couldn't tell how good the graphics were, but these excede Killzone's. Some comparisons:
The foliage especially is impressive in Halo. Greenery is one of the most difficult things to render, and it's gorgeous in that Halo 3 shot. I also prefer the colorful art style of Halo. Generic blacks and greys are overused in modern shooters. I much prefer things like Metroid Prime and Halo with lots of vibrant colorful environments. Both games look great, and I was certainly more impressed with Killzone at first, but looking back I'm stunned by Halo 3.