Besides the body shell, is one a better performer than the other? A series has better glass--larger optics. I believe it also has a larger sensor. the A series performs much better but the SD's are more pocketable and sleek. It's been my opinion (and I just repair them) that the A series is mostly an entry-level camera. It does what you need and nothing too fancy. They are nice though.
SD series use SD cards for storage and may have some extra bells and whistles (not much more).
Whereas the S series use CF cards for storage. I believe the SD is slightly cheaper than the S series (although I have not researched this).
In my opinion all of them are good quality. Canon doesn't make crap very often. Not to say that the other companies do, but I've definately seen some poor quality cameras with other brand names on them. | 