What is the best type of floor for a chicken shed?

I am having some concreting done around my home. We have an existing chook (chicken) shed that needs some renovating and it currently has a dirt floor. I want to put up a separate shed for storage of garden equipment next to the chook house and I am having a concrete pad laid for this shed.

Is it better to keep the dirt floor in the chicken’s house or would a concrete floor with straw on top be better for them? When I redo the chook shed, I want it to be the best environment for happy, healthy chickens. They will also have a large, fenced off garden area to range around when not in their shed.