Hulled are the easiest to digest and my chickens favorite food of all time. Unhulled are the cheapest and what most people feed. I feed both.
I've been feeding more of the hulled to one of my chickens that was half naked from a hard molt, to help her grow feathers faster. She went from being broody right into a molt and had a lot of skin showing. I wanted her to get some feathers in, before we got cold weather. She grew them back really fast. I also use them as a bribe or special treat. The unhulled are just food.
I give my birds treats of scratch, (oats, unhulled sunflower seeds, once in awhile bird seed, cracked corn etc.), in small quanities as these are treats. I also give them vegies, fruits, most everything that I can grow. I do grow a lot of corn and watermelon especially, also tomatoes, squash, root crops, the list can go on. I feed them everything except potatoes. I figure they will eat what they want. I do free range them.
Chickens do not have teeth. Their food goes, as is, into the crop, where it is slowly funneled into a very small " stomach" for some digestive additives--then to the Gizzard, where it is 'chewed', that is, ground into material that can be digested as it moves into the intestines and so on. The Gizzard is best able to break down whole grains and other chunky bits that they eat when full of grit. Longest lasting grit is Granite, that lasts well. All other rock and stone is so much softer, that it wears down fast and that is why granite grit is best choice, works really well for best utilization of feeds. My baby chicks are given free choice and they choose it with pleasure, baby grit is fine Granite, as soon as they are given anything besides Starter Crumbles. Their tiny gizzards are at optimum function at an early age.