I'm in the same boat as you are.
Not sure what to use.
Some say round, some say 2x4 on it's side, especially if in cold climate, as chickens will be able to cover their feet with feathers easier.
I might put both at same hight and see what they prefer.
Ask 10 chicken keepers and you'll get 11 answers, lol, because everyone has their idea of what is best.
I have 2x3 with the 3 inch side up, edges rounded
Chickens care less about the exact size/shape of the roosts, within reason, and more about the location. So any of the choices listed should work fine, or you can go with something more round or oval like a tree branch or a thicker wood railing.
They will prefer the top roost no matter what it is. I used small hickory trees. A top roost large enough to accommodate all the birds, or roosts all at the same height cuts down on fighting over spots and birds getting knocked off.
I have 3 different, 2 are round wooden rods, the 3rd is an old hand rail for stairs. From my experience as long as they can cover their feet and stay balanced, you are fine.