bike-fit

I'm looking to get a Trek Domane 4.3 and debating between a 54 and a 56. One shop I've been to told me I'm a 56 and another said 54. According to Trek's sizing chart I'm 56. I am 5 9" with an inseam of 30.5". Thoughts? Oh, btw I prefer not to replace any parts or make modifications to the out of...