Wire apron versus wire bottom. I don't have a tractor, I have a stationary coop, but it is dirt floored and I give the girls lots of grass clippings, pea hulls, leaves, corn shucks, pine straw, garden clippings and pretty much anything organic. What they don't eat, they scratch, poop on and compost for me. I dig it out several times a year and start over. Black gold.
My coop/run has a 2' wire apron all around it. I let them out in the yard in the evenings. I would think that a wire floor would make it harder to clean as chicken poop is not wire hole sized. Then you add in the shavings or bedding and at least in my opinion, it would be another chore for me to do-clean it out regularly instead of quarterly like I do now.
Wire apron, Marktoo is absolutely correct, predators will go to the edge of the coop/run and start digging. Then they run into the wire apron and can't go any further. However, they could tunnel under a wire floored coop or tractor and, depending on the size and strength of the predator, quite possible upset it and turn it over. I hope this helps.