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According to the geometry chart, it look like you're better off going for the 58cm as it's only 5mm longer, can be remedy by using a different handlebar with a shorter reach (says a compact with a 75mm reach, or 70mm reach).
Cannondale's measurement are fairly traditional, if you got a 56cm frame, expect a 56cm fit.
Having spacers is fine, you'll only notice it if you don't ride the CAAD10.
I think its the fact I dont want 30mm+ of spacers. The head tube is a small 155mm on a 56 Caad10. I dont bend that well.