I spotted Skyline Blue from NYC at CVS the other day, and it looked like it might be similar to the coveted Cinderella from Sinful Colors. So I picked up a bottle to do a comparison.
Skyline Blue is shown on my index and ring fingers, and Cinderella is on my middle finger and pinkie.
While these two are very similar, they aren't quite the same. Skyline Blue only took 3 coats, while Cinderella needed at least 4. Plus, Skyline Blue lacks the subtle pink shimmer that Cinderella has. They are very close in base color, and both are so slightly shimmery, but I would definitely say you could justify owning both. :)