Been reading this for the past few days, and have typed big long essays, and then deleted it all and decided to stay out, but today I am going for it
Having owned both, in the below order
Bought 40D
Bought 5Dmk1
Sold 40D
Bought 5Dmk2
Bought 40D
Sold 40D
Bought 5Dmk1
Sold 5Dmk2
Bought 5Dmk3
(leaving with me with the 5D1 and 5D3 combo)
I definitely have an opinion of 40Ds vs 5D1s!
In short, the 5D is a much better camera, despite its older tech.
In long....
Depends a lot which lenses you use. I primarily shoot primes, and wide open a lot. In this scenario the 40D (and possibly all crop sensor camera's) are a lot more susceptible to chromatic abberation and horrible purple-ness.
My favourite lenses include the Sigma 85mm 1.4 and Sigma 50mm 1.4 and Canon 35L - which all shine on the 5D. There is CA with each of those lenses, but only slightly, and under the right conditions you will get it.
On my 40D, these lenses were almost unusable unless stopped down to 2.8 or above, as the purple fringing and bloom (white bits having an almost radioactive shine to them around the edge) was terrible.
Using other lenses, the 40D was fine (70-200 2.8s, 17-50 2.8s etc) just not with those fast primes - which I loved, more than the camera.
Secondly, ISO. This is debatable, but the ISO noise I get at 1600 with my 5D is a lot more controllable than with the 40D. A high ISO 40D has a very digital looking grain pattern to it, which is hard to ignore. The 5D high ISO appearance is a lot nicer. Its very hard to get side by side examples of this and prove it, but if this is any help... I hated shooting above 800 ISO on the 40D, but on the 5D1 I will go all the way to H1 which is 3200! Dont get me wrong, it is still noisy at this ISO... but the noise doesnt take over the image, like it can do on the 40D.