The Advanced versions of the complex span tasks include higher memory set-sizes. This results in better discrimination between individuals at the upper end of the distribution of working memory capacity. This is ideal if you plan on sampling from a population that tends to be of higher ability. See Draheim, Harrison, Embretson & Engle (2017) for more information.
You are also given the choice of how many blocks of trials to administer. The options are 1, 2, or 3 blocks.