This question has come up a few times now. Sapphire on the left and Blue Moon Lagoon on the right.
This first shot is in the shade to try and illustrate the pink sheen that DS Sapphire has.
They are very similar in many ways. They are both blueish, the are both quite sheer, but neither of them are good for layering over anything as far as I'm concerned. They are sheer but in my experience they cover up whatever you layer them over quite a bit. Blue Moon Lagoon is obviously a bit more green leaning.
OPI DS Sapphire
OPI Blue Moon Lagoon
and now I must throw clothes on and run to work *whomp whomp*