Do it. Santa brought me a 2013 CAAD10 5. I'm 45 years old, and not a master yogi.

When I picked it up, the bike shop had it set up in comfort-mode, with the stem flipped up and the full stack of spacers. It was very upright and very comfortable. In fact, it was TOO upright for even me, so I went home and flipped over the stem. It's now much more to my liking.

I'm no racer, but I do like to go fast, and I like a responsive bike. But compared to my 1987 Cannondale SR400, the CAAD feels plush over bumps. But it somehow still retains that super-rigid rear-triangle that makes it feel so good to crank.

To be fair, I did NOT test ride the Synapse. If I were you, I'd ride them back-to-back. Don't worry about the lack of large sizes available. Even if the bikes are two sizes too small, you'll be able to feel the differences in the frame "feel."