My fav. eyeliner product is Make Up Stores cake eye liner, but I also have carbon and sometimes use it as an liner too. I prefer the cake eyeliner. Because it's desingned to use wet when you put your wet brush to it and swirl, the consistensy changes to this pasteish like stuff and becomes very easy to apply. With carbon I feel like I can't get enough prodcutc to the brush and the colour is more greyish as with cake eyeliner the colour is really black. When on eye the carbon looks powdery and the cake eyeliner looks like liquid eyeliner would look. So I'd say that if you want a softer look go with carbon and if you like a more dramatic look go with cake eyeliner.
Edit:
Oh, and a nother good product for lining your eyes would be MAC's Black Blac pigment. It's easier to work wiht than carbon gives you better colour payoff but dries to a powdery finnish.