Cannondale re-vamped the Synapse frames for 2014. They have received consistent rave reviews, and a lot of frame sizes sold out this spring. That's a factor that you have to consider, the new and improved frame. With the high demand, though, you're very unlikely to get a decent discount on a 2014. If you have a chance to test ride the 2012, and a 2014, you should. If the difference in the frame isn't apparent, then go with the cheaper one. It's still a very good frame, IMO.
I have the 2014 carbon 6, which is mostly 105. In the US, the 6 is lower spec'd than the 5. I wanted the 5, but I went with the 6 because the dealer had my size in stock, and the 5 was sold out and wouldn't be available until July at the earliest. The difference between the two 105 versions: Tiagra vs 105 brakes, slightly heavier wheels, and a house brand saddle instead of the Prologo. The one that matters most is the brakes. The Tiagra brakes are the worst that I've ever ridden. Worse than Tektro. The saddle was a pleasant surprise, though.
