CAAD10 sizes 48 and 50 are quite close, and they both seem to fit quite large. The top tube on the 48 is 51.5 cm, that's something you would expect on a size 50, and then the actual 50 is 52.5, so more like what you would see on a size 52 . Also, the head tubes are just 5 mm apart, and the seat tube angles are the same, so the only significant difference is the top tube length. You will be just fine on the smaller frame, even if you need to use a 110+ mm stem in the end.

[MENTION=324755]dudemanppl[/MENTION], what bike did you buy to replace CAAD10? I find the Cannondale race geometry to be quite aggressive, so I'm wondering what did you find that's significantly lower.