Cannondale is great in that they use stack and reach. It makes comparing sizes really easy, if you went with a 54 you would need 1.4cm longer stem and a 1cm of spacers.
A 52cm has almost the same reach as a 54 but another 1.7cm lower in headtube height(and as you pointed out, about .5cm less setback).
So I guess I would say, are you comfy on the 56cm? How many spacers do you have under your stem and is it turned down? What are you looking to get out of changing to a smaller size? At 5'10" you are borderline between a 54 and 56cm. I'm about 5'8.5 and I would fall sqaurely between a 52 and 54 I think. As there reach is the same, I would probably go with the 54 for the extra headtube height.