And don't forget that, with the introduction of the 580EX, the 550EX is now discontinued. Of course, you might find one used. I own both; the 580 does seem to give more consistent exposures with the 10D and the 20D than had the 550 given. However, both will work with any canon camera -- film or digital.
The 580, as noted elsewhere, is a bit more compact, does give more output, and gets more out of a set of batteries--at least these are my experiences.