I've had chickens off and on since 1965. My grandfather always used 2x4s on the flat for roosts, so that's what I always did. We've always had big heritage breeds, and even though they're birds, they're not 'perching' birds like little songbirds. The boards on the flat allow them to walk up and down the roost without having to do a tightrope balancing routine, so I imagine the wide roost is more comfortable. Also, I lived the first 48 years of my life up north and in the winter, the wide roosts let the chickens cover their feet when they roost, which I imagine is warmer than letting their toes hang out. Is it? I dunno. You'd have to ask the chickens.
The bottom line is, they'll roost on whatever you give them to roost on. I have my reasons for why I like 2x4s on the flat, but that doesn't mean I think they're superior to other roosts. Chickens want to roost and they want to roost up high. That's all they care about. Honestly, people stress about the shape of the roosts when what they should be stressing about is making sure the coop is dry, draft-free and has LOTS of ventilation. That's much more important than the shape of the roosts.