I have gone from a Rebel XT to a 30D and now to a 5DmkII. From the point of view of controls and handling, I found the transition from the XT to the 30D to be quite a jump. The transition from the 30D to the 5DmkII I found fairly easy. Obviously from a lens selection point of view, the 5DmkII is in a totally different class. You cannot use EF-S lenses on a full-frame camera, and if you want to get full benefit from the 5DmkII, "L" class lenses or equivalent are a must ($$$).
The 40/50d would allow you to continue to use EF-S lenses which are smaller, lighter, and cheaper, and I imagine you probably already have some. For your application, you may be better off with the 40 or 50d (faster shooting, smaller files, lighter, cheaper, etc). The 5DmkII, like its predecessor the 5D, is probably more suited for someone who's had considerable 35mm SLR experience and may be somewhat uncomfortable with the effect of a crop sensor on lens characteristics. I haven't had my 5DmkII for more than a week so I can't give you much information on it so far. But somehow I don't think I'll be using it much for snapshots (larger, heavier camera/lens combination). I hope this helps.
I do have a few shots from the 5DmkII at http://www.pbase.com/intermon
, if you're curious.