Round is normally not recommended unless it is a really wide roost, because birds can sometimes slip off.
Flat ones are good in cold climates because the chickens can lay on their toes to keep them from getting frostbite.
Also, you don't want the chickens' toes to be able to wrap all the way around the perch to the point of touching.
But your flock will want their opinions asked too...
TL;DR - the type of roost isn't as important to chickens as placement, so use what you want within reason (no slippery materials, nothing too narrow). I use 2" diameter tree branches because they're freely available.