personal experience from using 10D and D100 is as follows:
D100 has an immediate(0sec) startup time (from off to on to shoot). 10D has a 2+sec startup time. unless u want to leave the camera on all the time, its agonising to see a "moment" and missed it cos the camera couldnt fire immediately. Shutter lag for both are similar, but D100 feels more rapid, 10D feels lighter(shutter release, not weight).
D100 is lighter and smaller than 10D. its niffy to hold and use.
Quality wise, both are good. 10D's digic processor allows slight post processing on images b4 write (which generates slightly nicer pics off-hand). saves time if u dun like to edit pics.
D100 has longer battery life. takes almost double the amount of pics on 1 battery compared to 10D. depends on your lcd preview usage, 10D averages around 300-500pics for me, up to 1000 pics for D100. (less batteries to bring?)
SB80DX is lighter and smaller than 550EX, and uses only 1 button for both vertical and horizontal tilt adjustment (550EX uses 2). this further divides the 2 in terms of handyness. 500EX on the other hand, is already wireless ready, with transmitter on board.
on/off switch is at shutter release button for D100, at middle bottm of camera back for 10D. this adds lag b4 1st shot.
for events photography, D100 has an advantage of smaller combined size and weight, and most imptly, immediate response.
for non-immediate-response photography, 10D offers slightly more control.
btw, i own a 10D and love it
