I would say it depends on the type of snake. If it's an aboreal snake such as a tree boa, chicks and birds could make up more of the diet, since it's what they eat frequently in the wild. If it's a corn or a ground dwelling species, rodents should be fed most of the time.
That said, some snakes won't touch chicks after becoming adjusted to rats. I have 8 left in my freezer, since my Rainbow Boa ignored the last 2.
Paul