I still think a used 40D is a better deal than a new 50D. ...Has anyone compared the noise on the 40D to the 50D? Same sensor size, but more pixels often results in more noise.
As a physicist/engineer, I tend to research tech things before purchasing. So, when I purchased my 50D last year I also considered the 40D (and the Nikon D300). And as a 50D purchaser/owner I therefore must admit to prejudice, but...
As pointed out, both cameras share the same basic body design, viewfinder, 9-point AF, and both use the same external accessories like battery grip and wireless flash transmitter.
Continuous shooting is close enough to be a wash (IMO) at 6.3 fps on the 50D vice 6.5fps on the 40D. The improved processor speed on the 50D allows it to capture more images in longer bursts (JPEGs) when equipped with a UDMA memory card.
50D's pixel count is 15.1 Mpixels to the 40D’s 10.1. While the 40D has better high ISO noise performance on a pixel to pixel basis, once printed and/or downsized for the web, the 50D's final product is superior. As long as the final noise value is not degraded, more pixels are always the better choice for improved perception of image sharpness (technically speaking - up until slightly past the airy disk limit anyway).
The max ISO value for the 50D has boosted from 3200 ISO to 12,800 ISO, although the high end is of limited use (probably only in a news sense, not artistically).
The 50D supports in-camera correction of vignetting (on jpegs), and after the fact fix of RAW images in software (Canon's DPP).
The 40D lacks micro AF adjustment of lenses - a feature only on the 50D & above (so far).
The 50D’s has 640x480 pixels to the 40D’s 320x240. This allows for better use in Live View focusing (and IMO, better chimping).
The 50D has HDMI output for connection to a High Definition display - which works for playback of still images and "live" Live View. While I do not know the limitations, it is possible to capture the live view output and thus use the 50D as a very crude (probably studio only) video machine.
If you value these differences, then it’s worth spending the $$ for a 50D over the 40D.