They are variations of the same gene with differing degrees of smoothness. The smoothies are from the AC bloodline produced in the US, and Italian Leathers come from Alessandro's bloodlines in Italy. The smoothies tend to have maintained the same body structure and head type, making them pretty easy to distinguish. There are a lot of people passing smoothies as leathers these days, and they are definitely different to anybody who has owned both.
Crossed with each other they produce silkbacks.
It would seem so far that color is still needed in the smoothie lines, while the italian line has already produced some great color dragons. The italians have seemed to be smoother, but that is changing as the the smoothie lines get refined and are closer in quality.
Jeff
5.3 Italian line
0.4 AC line