All of the above and...gray's are meaner. Had both as pets. You cannot tame a gray, but a red actually makes a pretty nice little pet. They just don't like you to grab them and pick them up like you would a puppy or cat. Grays will take refuge in a hole in the ground but they don't den. A gray will just go out in thick cover and lay down, even to have pups. Reds, at least the ones around here, almost always den and whelp pups in an old groundhog hole. Grays of course can climb a tree and that is why reds were brought here from Europe in the first place....they can't.
I always heard that gray's and red's wont live together in the same area and that the gray will always kill or run off the red's. I don't know if that is true because in the last few years I have had several trail cameras set continuously and see red's and gray's often within the same hour. That is both in Md. as well as the mountains of Va. I even have pictures of coyotes and fox in the same hour...of course the coyotes might have been hunting the foxes.
Red's smell similar to a skunk and you can smell it in the woods if one is near. Lastly, I have never seen a gray fox with the mange but have seen many red's that had not a hair on them because of the mange.