The big visual differentiator between the two games comes down to the handling of transparent "alpha" textures. These eat up bandwidth and fill-rate on the consoles, and as regular Digital Foundry readers will know, the 10MB dedicated RAM attached directly to the Xbox 360's Xenos GPU can give the Microsoft console a very real advantage here.
A very common solution on PS3 is to reduce the resolution of these textures: Killzone 2 for example scales them up from a quarter-resolution buffer, but adds multi-sampling anti-aliasing to smooth off the edges. For effects that are on-screen for a split second (for example, explosions) it's very hard for the human eye to notice much difference: it's a massive bandwidth-saver, with little impact on overall image quality.
BioShock 2 employs the same trick with its transparencies (without the MSAA). A massive amount of the game's alpha textures are rendered with a quarter-resolution buffer, which is fine, except for one problem: these aren't on-screen for a split second, they are there a lot of the time. All of the water, particles and fire effects in BioShock 2 are rendered in this way, meaning that depending on the scene, some or even all of the screen is being generated at quarter-HD resolutions.