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Either will work fine as long as it's wide enough so they don't have to grasp it with their toes (since their feet are really not designed for this). A round dowel type roost would not be the right thing for chickens. If you live someplace that gets below freezing in winter, make sure that when your chickens are on their roost they can cover their toes with their breast feathers to prevent frostbite. If toes are sticking out, your roost is probably not wide enough.
I've used both and my chickens seem perfectly happy with both. It's easier to get a piece of lumber smooth and sanded than a tree branch, which makes it easier to avoid sharp spots that could injure feet, and makes it easier to clean. On the other hand, you can just throw away tree branches periodically and replace them with fresh ones when they get nasty, since they're free. Tree branches can harbor mites from wild birds, so before you use one it's a good idea to douse it thoroughly with something like Adams Flea and Tick spray then put it out in the sun to dry completely.