Frame Size Trek Emonda SL 2021

Hi everyone,
I want to buy my first proper roadbike and atm the Trek Emonda SL6 is my favorit.
question is which size to buy.
I’m 183cm (6’0") with an inseam of 86cm (33.8") so arround 76cm (29.9") seatheight.
Treks website suggests to buy the 58 wich would come with the longer seat mast and a minimum rail height of 72.5.
On youtube i saw that the 56cm frame (coming with 70.5cm minimum rail height) has a minimum seathight of 74cm and led to him swapping the seat mast to the shorter one.
So im wondering if i’m goingt to need the shorter seat mast with the 58cm frame too. Or if i should buy the 56cm frame instead.

For comparison: my Tri/TT bike is a BMC TM02 in Size M-L with 515mm stack and 418mm rach.
Any tips?

I am very much a 54 in a Trek (current bike: Madone SLR). I am 177cm with slightly short legs (80.5 inseam), and run a 71cm saddle height (bb to saddle top). Trek measure to the saddle rail, as you note, which is c.4cm below actual saddle surface height in most cases. I am right at the top of a short seatpost.

I would be fairly confident that both a 56 and 58 could accommodate your saddle height, so it would be a case of which stack and reach you prefer.
reach/stack for the 56 and 58 respectively.

Thanks for the advice.