oil slick is more like black tied, although they're still very different, imo. black tied is much easier to work with than oil slick, and the sparkles are more subtle. its also easier to blend.
carbon may be better for a well blended, gradiant smoky eye, it blends like a dream, once you get the hang of it. i have both carbon and black tied, and i reach for them both equally as often. i couldn't imagine not having either of them. but if you have oil slick, and can only get one, i'd say go for carbon.