Image Quality
There seems to be this preconceived notion that DSLRs have better resolution than compact digital cameras. This is probably true most of the time, but not in the case of the XSi and G9. The Canon PowerShot G9 actually outperformed the DSLR in terms of resolution: check out the vertical and diagonal lines above the “24.” They are better defined on the 12.1-megapixel G9 even though the XSi claims 12.2 effective megapixels of resolution.
 

Canon PowerShot G9	Canon Rebel XSi

Canon PowerShot G9

Canon Rebel XSi

Perhaps the rift in the resolution is due to the differing imaging sensors: the XSi has a CMOS and the G9 is equipped with a CCD.

Here comes another DSLR-compact digital camera stereotype: compacts regularly oversaturate colors more than their DSLR counterparts. This is true in this case. The Canon EOS Rebel XSi’s colors look dull next to the G9’s. See for yourself: the pictures below show incredibly green grass and a flashy red sculpture on the G9. The XSi keeps colors true and the G9 makes them look brighter and fuller than they really are.
 

Canon PowerShot G9	Canon Rebel XSi

And if you look at these pictures of a Frank Gehry structure, the G9 has a cooler steel color than the XSi’s warm tone.

Canon PowerShot G9	Canon Rebel XSi

If you look at these pictures taken of a color chart, the colors come out differently depending on which camera was used.

Canon PowerShot G9 ISO 80

Canon Rebel XSi ISO100

On the XSi, the black is blacker. But the white is whiter on the G9. Each color seems to have its own variation. The G9 follows in the footsteps of the PowerShot lineup with its oversaturated colors; this is typical of compact digital cameras because it brightens up dull colors and pale skin tones to look better in pictures than in reality.

Let’s talk about noise. The Canon XSi has cleaner images than the G9 throughout the entire ISO range. You can see just how clean the images are in these pictures of skyscrapers: look closely at the windows of the buildings. 

Canon PowerShot G9	Canon Rebel XSi

This may be a subtle difference, but the biggest differences in noise production come in the dark. Turn down the lights or go outside at night and your Canon G9 will be as useful as a flashlight with no bulb. The Rebel XSi, however, is well-equipped for night photography.

Canon PowerShot G9 - ISO 100	Canon Rebel XSi  - ISO100