My fleet of bikes has now reached 8. Some have SPD, some have Look, and some use clips and straps. As mood strikes I switch them around. If I was going to race (yeah, right
) I would use the Looks because they seem to transfer power more efficiently due to the wider platform. If this is important to you, then I think you will see some improvement over the SPD's.
Since I stopped caring about things of this nature, I prefer SPD's because of their convenience. I like to be able to use mountain bike shoes on my road bikes at times.