According to Beyond3D, the PS3 version of Tomb Raider Underworld runs at a higher resolution than it's Xbox 360 counterpart.  The PS3 version runs at 1280X720, the standard 720P high definition resolution, while the Xbox 360 version of the title runs at 1024X576, a sub HD resolution that is then upscaled.

What does this mean?  The PS3 version is HD and the XBox 360 version is not.  However, the PS3 version does suffer from the usual 'washed out' lighting effect.  You can see screen shots and make your own comparisons over at the Beyond3D forums here.