So I've been very interested in comparing the Canon 50D with the 5D recently. Both are good, but very different cameras.
A quick run down, the 50D has more megapixels but the 5D is better quality. 50D has more functions, 5D is supposed to handle better at high ISOs. A big difference is the image sensor size and the lens compatibility... 5D's do not accept the EF-S lenses, 50D's do. While all of this might be foreign to most of you, I'm doing these tests because if you are familiar and have done your reading in the forums, reading only gets a visual person (like me) so far. Here, visual examples. And if you have no clue what I'm talking about, then a nice little visual example might help you to at least slightly understand what I am talking about.
50D test images are on the left, 5D on the right. Top 2 are taken with the SAME 50mm lens at the same distance from the subject. The 50D has an image sensor size of 1.6X. The 5D has a 1.0X, or full frame sensor. The full frame will make the 50mm lens shoot at 50mm, but the 1.6X frame with the same 50mm lens will shoot with a FOVCF (or crop).
(can click on image to enlarge)
The middle & bottom images were taken with the same 8mm fisheye lens. On the middle left is a straight shot from the 5D. The bottom left is that same image cropped in photoshop to mimic the image on the bottom right from the 50D which is a straight shot from that camera. The image on the middle right is the same lens, on the 50D, with the hood on the lens to give a sharper & more defined circular edge.