I'm a little bit late to this party, but am considering a Zektor 3 myself. I'm 5'9" with long limbs, considering a 56cm size. I currently ride a 54cm Cannondale Caad8 road bike and it's just slightly too small. I'd say I'm more of a true 55cm.

Would you, in my shoes: order the 53cm or 56cm?

ETA: The LBS doesn't have either a 53 or 56cm model in stock for me to try out.