Flash
Both cameras have a hot shoe that accepts Speedlite flash accessories, although the bulkier XSi will hold it better. The flat and heavy G9 will be awkward with a flash the size of itself stacked on top.

The G9’s built-in flash is typical of a compact digital camera: the flash is set above and to the side of the lens, making for uneven lighting. The flash looks good when used during the day as a fill flash to compensate for shadows. But when it’s the only light source, pictures will look like something from a horror flick.

Head to head, the Canon XSi easily has a better flash. The XSi’s flash pops up directly above the lens and sends out lighting that is much more even.
 

Canon PowerShot G9	Canon Rebel XSi