I'm looking at getting a new road bike, specifically a Domane, and was recently fitted in order to narrow down the size. My fitter recommends that I get a 54, specifically because I need the steeper STA to get my fore/aft in an acceptable position. He also said the 54 will allow me to lower my bars some in the future. On the 56, my stem would basically be slammed if I set the bars to the same height as on my current bike.

I've read over and over again to size the bike by the top tube length, but the reach difference is only 3 mm between the 2 sizes, and STA is only .4 degrees. From a fitting perspective, am I correct in thinking that main difference between the two is stack height, and how high or low my bars can be fitted? I want to make sure I don't get it wrong in regards to reach if I get the 54.