Hmmmm. The great 40D vs. 50D debate rears its head again
I started with a 450D, and, although a great little cam, I found the ergonomics and versatility a bit limited, and soon wanted to upgrade to either a 40D or a 50D. Fortunately for me, I had friends that owned both, so was able to get a bit of hands on experience with the pair, which even allowed me to do a bit of pixel peeping while I was at it.
When it gets down to it, the picture quality of both cameras is too close to call, with the 50D giving you the ability to crop more, though. Most of the comments people make about loss of picture quality due to having more pixels "crammed" into the sensor are proved nonsense in practice, as the sensors for the 40D and the 50D are completely different designs.
The ergos are exactly the same for both cameras, however the 50D is a bit more up-to-date in terms of bells and whistles, which do make for a more versatile camera. Check out the differences here:
http://www.dpreview.com/reviews/canoneos50d/
I think, if I were buying earlier this year, just at the time the 40D ceased production, the price differential between the two would have pushed me towards the 40D. However, right now you can get a good secondhand 40D for between £450 and £550, and a 50D for £550 and £650, with the latter still being in production. So, at those prices, to me it really was a no-brainer; you're not going to get much glass from the saving you could make by going for the 40D at that rate. And I have to say I am very pleased with my choice! I also feel that if I'd gone for the 40D, I would be hankering after an upgrade sooner, whereas I'm quite content that the 50D will last me a good while yet.