Both wines have a number of things in common: they are complex, provide a seamless taste experience, they give great mouthfeel, are elegant and have excellent structure. Both are quite big and full flavoured.
So what are the differences? Based on a few experiences, and now repeated, there are mainly two: the Giaconda has a predominant grapefruit flavour profile, whereas the Leeuwin tastes more of peach and nectarine. The second difference is the more significant creaminess in the Giaconda, probably due to more substantial new oak treatment. In the Leeuwin, the fruit stands out and the oak is very much in the background.
In summary, you could confuse the Giaconda with a top flight Meursault Burgundy, the Leeuwin tastes more uniquely new world.