If you haven't been riding regularly, my guess is that just about any road bike is going to feel nervous at first. The Synapse and CAAD frames are pretty different in terms of geometry. These bottom-end Sora-equipped bikes are pretty much identical in everything other than frame geometry. The CAAD8 has a more aggressive, race-inspired geometry while the Synapse has a slightly more relaxed geometry. If you're looking for stability the Synapse is the way to go, though the wheelbase is a bit shorter than other relaxed-geometry bikes (ex: Specialized Secteur).

Edit: my recommendation would be to avoid Sora components. Shimano's Tiagra-level components work much better and are worth the extra money.