I did start w/ medicated because that is what the clerk sold me when I bought the 1st bag. I have read that either is fine, but the medicated helps the birds build up a tolerance to a normal GI bacteria (cocci) that is present in all chickens. But like E. Coli in people, if there is too much of the bacteria it can sicken and kill the chicks. Older birds have better resistance to the problem, but they developed the resistance as they grew. If older birds are near to young chicks, they can introduce high levels of cocci and this is BAD for the young ones! Being outdoors on dirt can also help build their resistance to the effects of cocci.
My chickens are pretty much raised au natural now, but I did start them out on medicated starter. Coccidiosis can kill a tiny chick so fast, almost before you can detect the illness and treat it. I consider the medicated starter to be an inexpensive and simple insurance against heavy losses from it.
I started mine on Medicated. They got one small bag of that and then on to regular chick feed. This is what I was advised to do. It didn't take them long to eat up all the medicated food and they all seem to be doing fine.