You should be able to make either size fit exactly the same as long as you don't require a position that is extreme. Meaning if you get the 54, you can make it fit just like the 56 as long as you have enough seatpost, steerer tube and the right stem. At one extreme, you will be able to set up the 56 for a more upright position; at the other end, the 54 will be capable of a more aero position.
I am your height and have a 33" inseam. I currently ride a 56cm traditional geometry frame with an 80mm stem, no spacers and ~6.5" of saddle to bar drop. The 80mm stem handles fine for me and after the initial few rides I don't notice a handling difference from the longer stem. If I were to buy a new frame now, I think I would get a 54 or 55cm depending on the sizes available because the 56cm will keep me from going lower in the front if I determine it would be advantageous.
If you are as flexible and lean as you plan on becoming or are going to be riding primarily for recreation, then the 56cm frame is fine. If you anticipate that you will become more lean and flexible or you are interesting in racing, you might consider the 54cm frame.
Also, I'm sure it's imperceivable, but a smaller frame will weigh less and be stiffer.
Originally Posted by
TromboneAl
I couldn't pass up a good deal on a 54. I've got a long stem and seat post and it fits great. My last bike was a 58!
Attachment 175674
I'm 5'11" with 31 inches floor to crotch.
This isn't what we are used to seeing, but it's a good example of what I'm saying. This might not look "normal" and I bet the 58 looked better when fit for your preferred position, but the top tube would have been in your crotch when your feet were on the ground (unless your reported inseam is wrong).