The variety of soil substrates is a great one. Basically, soil provides your aquatic plants with the very best basis for healthy growth and adapts your tank water to the requirements fish and shrimps from soft-water habitats need to breed. Aqua Soil Amazonia by Aqua Design Amano is especially rich in nutrients. Environment Soil by GlasGarten makes your tank water extremely clear from the very beginning and is especially sought after by shrimp keepers and breeders. This soil brand is also highly suitable for planted tanks.
Soil substrates are offered in various granule sizes. "Powder" type soils are very fine-grained and make planting considerably easier. Moreover, in nano tanks this type of soil looks very much to scale. However, it is also recommendable for larger aquascapes. There it is used as uppermost soil layer, where it enables ground-covering plants to take root more easily. Their delicate roots have a better hold in this substrate, and the plants have less tendency to become buoyant. The standard grain size is recommendable for larger aquascapes where more soil volume is required.
Advanced Soil by H.E.L.P. is a hybrid, with fine as well as coarser granules, a so-called multi type soil. Aquatic plants take root very easily in this mixture, and you do not need as much soil to get the same volume as with a powder type.
The most prominent characteristic of soil substrate is its ability to maintain a shape for a long time, e.g. if you want your substrate to form a hill or a small mountainscape. Sand and gravel will flatten out again pretty soon, whereas a slanting hill moulded with soil will remain stable for a long time.
All in all, soil substrates are the best choice for an aquascape, and planted tanks in general.