There is no real difference between the two, it's down to the other pieces of kit.
http://www.engadgethd.com/2007/09/13/hdmi-vs-component/
There are plenty of otherarticles available if you google.
@Garcian
Are you saying that optical/digital coaxial CAN'T do 5.1 audio? If so my AV reciever must be lying to me.