I honestly can't see a difference. The formula is pretty much the same as well. They are both on the thick side but not too thick and both only needed two coats. They are each a lovely grey/silvery blue with subtle shimmer. I'm also amused that both are beach/sea/water themed. Sag Harbor may look a bit darker but it isn't. They are pretty spot on to each other.
If you have been wanting Essie Sag Harbor, but haven't been able to get it since it was limited edition, China Glaze Sea Spray may help with your lusting.