Like everything to do with chickens you can get a lot of different opinion on this. People may state reasons why they like one way or another but it’s basically opinion.
I once switched roosts, using tree branches, 2x4’s flat, and 2x4’s on edge. Mine like certain areas of the coop (all things equal near the window is the prime roosting spot) and they like to roost on the highest spots available so I took that into account and really switched them up. My conclusion was that people care about the shape of the roost much more than chickens do. I encourage people to do a similar test, taking into account that some spots are preferred and they like higher spots. Draw their own conclusions.
Some people argue that a chicken’s foot is made to grasp a round tree branch so round is essential. Some argue that a chicken’s toes don’t bend so they can’t grasp, they just balance, so a flat surface is best. I saw one where someone said chickens did not evolve to roost, they evolved to sleep on the ground so they need something flat. Some of these discussions can get pretty heated.
Some argue that a chicken needs something flat so their feathers can cover their feet when they sleep. I use tree branches and 2x4’s on end. When my chickens settle down to sleep in cold weather they fluff their feathers and their feet disappear. But my winter temperatures don’t get that cold, usually just a few degrees below zero Fahrenheit.
If you provide something round some people will think you are great and really love your chickens. Others will think you are abusing your chickens. If you provide something flat some people will think you are great and really love your chickens. Others will think you are abusing your chickens.
My suggestions are that you use wood for your roots, not metal or plastic. Wood is a good insulating material, metal or plastic are pretty good conductors and can wick heat away from their feet, increasing the risk of frostbite in weather below freezing. If you use sawn lumber like 2x4’s, I suggest you sand off the corners. That’s not so much to make it easier for their feet to grasp the roost but to remove splinters. I don’t have a suggestion on shape, in my opinion they all work.