I am leaning toward the 56 cm frame at the moment since less drop from seat to handle bars appeals to me.

But it would seem this would also depend on the bike geometry. Looking at endurance and gravel bikes that already have more relaxed geometries and longer wheel base maybe a smaller frame would work. I wonder if a 54 cm frame on a Roubaix might fit similarly to a 56 cm frame on a Tarmac.