How tall are you and what's your inseam? I'm a little over 5' 9" and have a longish 32.5" inseam. I'm also torn between the 54 and 56. I was thinking the 54 might be better. Problem is I don't know if I can get the bars level with the seat, my preferred ride.
I'm 5'11 and my inseam is 32, on jeans at least. I'm not sure what it is technically. I clear the 56's stand over height of 31.9 by about a third of an inch, not much. I would focus more on the eTT because the saddle is much more easily adjusted than the reach. A rule of thumb about stand over I heard is that you should be able to clear the top tube by 1 in to 1 1/2 in. Someone correct me if that's wrong! If I were you, I'd think about a 52 even. a 52 and a 54. The 56 is likely too big. The LBS said if you normally ride a 56 on any other road bike, you should get a 54 on the CC because its top tube runs long. Good luck.