Ridgerunner makes good points.
It's all up to you
My birds hate round ones as soon as they get big enough to roost on 2x4s, with the 4" side flat. Their little chick feet loved the round small branches (dowel sized), but as soon as they start noticing that the big girls get a big board higher up, they start climbing up the "ladder" per se, and they tend to stop using the branches and prefer the boards, just because my roost is set up that way; they will almost always go for the top of whatever roost there is.
Here's a snapshot of one if my houses, the roost is angled almost like a big ladder leaned up against the wall. The boards are 18" apart going up, and 8" apart on the lateral going in. I found one that also shows the area I separate to brood chicks, and it has the smaller branches. I also turned the 2x4s on the roost after they had outgrown the 2" side, so now they look like this...
This was when it was first built, with the brooder.
But then again, I have free rangers that prefer the trees, and those are most definitely round branches
Try a little of both and see what they like; but if you're in an area that sees cold winters, they would appreciate wider roosts so they can tuck their feet under their bodies at night to keep them warm, no toesies getting frostbit hanging over onto a roost
