Why do you think Canon came with so many models not significantly different from each other with respect to their performance.
EOS-1D Mark II N
EOS-1D Mk III
Uhhh... the 5D and 1D Mark II N and 1D mark III are nothing... NOTHING... alike.
The 5D is a nice, full-frame prosumer camera. It snaps 3 shots per second, 12 MP. It has nice build quality, but it's not hardy in the rain like the 1 series cameras are. A few folks had their 5Ds crap out in Antarctica during a 2 hour shoot in the rain.
The 1D II and 1D III are professional sports cameras. The II does 8 fps, the III does 10 fps. The difference between 3 and 8 or 10 fps for sports is... COLOSSAL.
The performance differences for these cameras could not be greater. If you want to do landscapes, on a budget compared to the 1Ds mark II, you should go with the 5D. If you need speed, because you cover sports, auto racing, or whatever... the 1D is the camera. I have one and it's fantastic for how fast it is.