With the recent announcement of a $200 rebate now available for the Canon 40D, it is necessary to do a side by side comparison of the two cameras because they are so close ($240) to each other in cost. I own both cameras and know them inside and out, so I thought I would provide this chart for your consideration.

Camera Model
Current Price (BH Photo)
Max Frames per Second
ISO Range (including Expansion)
Max Shutter Speed
Body Construction
Custom Mode Dial Sets
Max Flash Sync Speed
Secondary Selector
ISO Selection Increments
Timer Differences
Menu Differences
Other Significant Features
Canon Rebel XSi
12.2 MP
Plastic, Not Weather Sealed
Directional Pad
16 oz.
1 Stop
Continuous Shooting
Highlight Tone Priority
Canon 40D
10.1 MP
Magnesium, Shock/Water Resistant
Wheel and Joystick
26 oz
1/3rd stop
10 Second Timer
Loaded to the Gills
Top LCD Readout

I think the extra $240 is worth the body alone for the 40D, but throw in all the other goodies, this is a no brainer. I do not see the extra pixels enough to save the XSi in a head to head competition. That said, the XSi is a sweet and very light camera, the extra $240 would be great to put towards a new lens. My suggestion is, if you can swing the extra cost, go for the 40D, if not the XSI is simply awesome.Whichever you decide, just know I have both cameras detailed and described in two fantastic DVDs:

Canon Rebel XSi Crash Course

Canon 40D Crash Course DVD Training