I've hunted and trapped foxes quite a bit, their ranges overlap and although typically smaller Grays are a much more agressive animal and will be the dominant species in an area where they share habitat.
I have found that Grays will live in a more diverse habitat, everything from your typical low farmland, swamp, brushy type areas to big woods. I have always just associated Reds with farm country, Grays can be pretty much anywhere.
Diets are similar, builds are different and grays are quite a bit easier to call due to their agressive nature. I have also found that Reds seem to like to hunt mostly at dawn and dusk where Grays from my experience are a pretty strict after dark hunter. I'll have my best action on reds during the last or first hour but grays we will call all through the night and the darker the better.
Grays can also climb trees and bark when they know the jig is up. I've had them hang up downwind just barking their heads off at me. LOL
Cool animals, I always loved foxes.