I am in the market for a new road bike right now and I am shopping around to see what I want to buy. After a few test rides and thinking that something with a mor relaxed geometry would be more my speed. The Cannondale Synapse is a well known example and I have been on one. A few weeks ago Competitive Cyclist had the BMC Road Racer with the Di2 Ultegra group on sale for $2500 and the price really caught my eye. Since then, I've been mulling a bit over whether should change my mind about what I'm looking for and then I really examined the geometry chart on the BMC. Now I have a question that I hope someone here can help me with.

I have been measured and I look to be a good fit on a 56cm Cannondale.

Here is the geometry of the Cannondale:
http://www.cannondale.com/2013/bikes...mpact-crankset

Here is the geometry on the BMC:
http://www.competitivecyclist.com/fr...012-10666.html

The 57cm version of the BMC seems to have a similar set of measurements to the Synapse although there are some differences. How much of a difference would there be between the fit of both bikes? In my totally amateur intepretation, they look to be similar enough that a shorter stem could almost make them ride the same. Am I way off base? If so, what am I missing?