What part of the image characteristic do you prefer on the 5d?
The 5D still puts out a sharper image than the 40D or XSi. It appears able to resolve a hair more detail. I can see more veins in the leaves of a tree from the 5D images. Although I must admit that between the 5D and the XSi I get a bit cross-eyed in the review because it's tough to see the difference. The bigger difference between those two is the quality in higher ISO and the 5D definitely takes the prize in that comparison.
There's still a 3D look to those 5D images that isn't there as strongly in the APS-C camera shots too, although that's probably just a DOF difference. When I put these feeling into words however and I read them back it makes the cameras sound like they are further apart than they are. It's still quite possible that what I saw was partially subjective and there are inevitable variations in exposure that can make a shots looks different enough so that it can be hard to tell about the differences in detail. These three cameras really are very close imagewise and one should not feel disadvantaged by owning one as opposed to one of the others.
The one thing that made me choose the XSi and 5D combo and sell the 40D was that I took the 40D out on several treks and shot landscapes with it and just never got much that I was happy with. While some of my best work is from the 5D and lately I've shot some satisfying landscapes with the XSi as well as some great sports shots too, despite the claustrophobic buffer and shutter firing that feels like it's moving through molasses.