Concrete or Dirt?

I have built my pens 16'x16' and 8' tall used cattle panels and lined the floor with cattle panels then poured p-gravel 4" thick they still try to dig a little but a garden rake will fill the holes easy and I use a stall rake to pick up the poop easy to clean and urine drains through the gravel they seem happy and have plenty of room to run around these are by far the best setup to date I have had I still use a chain on a couple of male dogs that only because I have 3 pens and need to build 3 more. I plan on running a hot wire around the top because I have one gyp that will climb out shes a pain in the ass.