I'm a fan of the slate, although my current case looks identical to the new space gray. I picked up 3 interchangeable back plates so I can switch them out every so often. I am using the one in the middle right now.
I decided I would get the darkest color they offer, The Space Gray. Seeing it next to the Slate makes me long for it. However, there is nothing wrong with the Space Gray.
My only question is, since they are using the gray back and the aluminum sides will there still be a scuffgate situation with this color although it's not black? Should I expect to never see nicks or scratches on the back and sides of the iPhone 5s Space Gray? Or if there are scratches and nicks would they be virtually invisible because of the exposed aluminum?
I'm thinking the back is not slate but it is not impervious to showing damage because it is still a darkish color.
The only reason it may matter for me would be for the resale value (people may be partial to a particular color). I could not tell you what that color may be though. I know people care about color despite hiding it behind a hideous case.