They're both ultegra the 6600 was called ultegra sl it wasn't any lighter than the standard 6600 but it was dark grey. It the previous generation ultegra to the 6700 and the rear mech has a slightly smoother less angular body to the 6700.
6700 top
6600 bottom
They're both ultegra the 6600 was called ultegra sl it wasn't any lighter than the standard 6600 but it was dark grey. It the previous generation ultegra to the 6700 and the rear mech has a slightly smoother less angular body to the 6700.
6700 top
6600 bottom