I go with the cheapest grains that they will reasonably consume....I've found that no chickens like cracked corn anymore. I'm assuming it's because it's all GMO now. I stick to barley, oats, a little cracked corn, etc. Just choose what you can afford and what the birds will not pick through. Each flock is different.
For my meat birds, I give layer ration as the base and just mix in the whole grains.....I want to slow down their growth and save money on feed, so I free range and feed the 50/50 mix. If your goal is to crank out some meaties in a hurry, then you would stick with the higher protein feeds...but I wouldn't recommend it. If the reasons you are using FF is for health and costs, feeding high pro defeats that whole purpose.