When I was looking at new bikes a few years ago, the 58cm frame felt better for my 5'-11" height and 34.5" inseam than the 56cm of the same model. However that was just sitting on them in the shop and a ride around the parking lot. After riding each for 10 miles, the 56cm won out for quite a few reasons.

Wider bars on the 58cm bike braced me too much against side to side sway and my shoulders took all the brunt of the force.
The 175mm cranks were just way too big since I am use to 165mm cranks. I felt like I was thrashing at high cadences and it was uncomfortable for the more aero position I desired. I was felt worn out after those first 10 miles, but the second 10 miles on the 56cm bike and I recovered and felt ready to do 20 or 30 more miles.

While you can make a bike of most any size fit you well, you might have to change a lot of stuff to get it there. Cranks, bars and stems can start to add up. So if you can, try to ride the bikes as far as they'll let you. The bikes I rode weren't the bikes I was actually going to buy, but they were the same frames with lower tier group sets and wheels on them.