Totally and completely incorrect
I guess I am wrong about the height issue.
As for the power though, you have to be careful to compare guide numbers in the same units; you used meters for the built in and feet for the 220. The comparable guide numbers are 13m for the Rebel XT and 22m for the 220EX. That's not a huge differencem particularly in comparison with flashes that have guide numbers of 43, 50, and 58 respectively.
The 220EX does do autofocus assist without relying on strobing the flash, which is nice. So do all of the other add-on flashes being discussed here.
The 220 doesn't do E-TTL 2, which is bad. The Rebel XT (but not the older Rebel) built in flash does. That means that for a Rebel XT owner using the 220EX means taking a significant step backwards in flash metering capabilities compared with just using the built-in flash or even off brands that support E-TTL 2. So while it does talk to Canon's "in their own language," it uses a now outdated form of that language. At the same time, it looks like Sigma has taught their flashes to use the latest Canon language.
As agreed, it lacks bounce. To me, this is a critical ommission.I would consider that an
essential feature in any flash where the user intends to take pictures indoors.
Using direct flash is usually a last resort for me when all of my other lighting options have been exhausted and even then I try to minimize the amount of lighting that comes directly from camera. The reason is that lighting your subject directly from the camera eliminates all of the shadows that we use to perceive depth and texture and we are left with a very flat looking picture. With a typical indoor flash photo, I'll bounce the flash from the ceiling to illuminate my subject with a diffuse light above them. I'll also bounce some of the light from the flash directly at the subject to help reduce the contrast in the shadows. This is done with the built in white card on some flashes or by holding a white index card at the back of the flash with a rubber band.
As for price, the Sigma is about $30 or $40 more than the 220EX. That's not a large dollar difference for a flash that can bounce, has more than twice the guide number, and has E-TTL 2 metering capabilities. My personal preference is usually to buy Canon and not worry about any compatibility issues, but if I were choosing between flashes and didn't want to spend the money to get a 430EX or 580EX, it would be a simple decision to get the far more capable Sigma.
With that said, I should point out that I have no personal experience with the Sigma flashes and am relying on their claims that they support E-TTL 2 appropriately. Still, for me personally, I'd rather go back to a manual flash that bounces than shoot with a fully automated one that doesn't. It's that important to me.