They are virtually identical but the 550 has a little more settings you can play with. SD550 is also 7.1 megapixels but the body is slightly larger. The body is larger because it has a bigger lens. The image quality between the two is basically the same but I thought the 550 is a little better. All the reviews I read are comparing cameras to the 550. That eventually swayed my decision.
If you want to know everything possible about the review of a digital camera just check out http://www.imaging-resource.com/
Too much info. makes my head spin