I have the 566, if that helps. I have the stereotypical "female" build with long legs/short torso and found that it only needed a shorter stem (9.0 cm instead of 12) to make the 53cm size fit perfectly. (I'm 5'-8" with a 30.5" inseam)
Is it possible for you to try this bike out? Looking at the geometry chart in the catalogue for a 51cm frame, the 586 has a very slightly longer top tube (about 4mm, or less than a 1/4") and frame angles of 74.25º seat tube/72º head tube instead of 74.5/71.5.. Not a lot of difference.
My only reservation about the 586 might be the seat post. It looks like a one-off, and I wonder if it could be replaced easily if it ever broke. I don't think you'd be unhappy with it, but then I'm biased. My Look feels like it was made for me.