Here are screenshot comparisons of Gun on both systems, using a high definition TV. Note that of course if your COMPUTER MONITOR is low resolution, you won't see the detail difference. The screenshots I show can only be as good as your monitor is.
I took these pictures with a digital camera. Note that my hands aren't perfectly steady, so there is a little jitter here :)
Regular XBox version of Gun, pause screen
XBox 360 version of Gun, pause screen
This pause screen is just when you get on the steamboat and are able to start fighting. Note that "live action" is paused here - that's not a painting you see. You can see pretty clearly how the XBox 360 gives you a wider angle here - on the right of the XBox the blue box is right at the edge of the screen, while at the XBox 360 there is space past it to the right that's visible.
XBox 360 Comparison Screenshot Set 1
XBox 360 Comparison Screenshot Set 2
XBox 360 Comparison Screenshot Set 3
XBox 360 Comparison Screenshot Set 4