If you are planning on buying a high end road bike, or any road bike for that matter, get a professional fitting. Systems like the Serotta fitting system are able to dial in the geometry and tube lengths on their fitting bikes so you don't need the actual bikes. The extra cash for a decent fitting is well worth it because not only will you know if the 56cm or 58cm is the better choice, but you'll also have a very good idea of stem length, stem angle, handlebar height, and seat height.