The 54 is 54.8 cm (55 in round numbers) in length and the 56 is 56.5 in length. The seattube is not important because the bike has sloping geometry. Based in the length which one the OP would pick? I'm like the op and personally i would pick the shortest one because it will easier to fit than the next size. Any ways, thats my personal opinion but the op always can pay for a fitter to tell him what to do