Topsoil typically has a great consistency with soil and organic matter. It brings a good microbial complex with it which holds and converts nutrients well. Topsoils also tend to drain well. The downside is that they contain weed seeds. Topsoil by its nature is a surface soil and the only way to completely eliminate weed seeds is solarization which strips the microbial value from the soil. While topsoil is a great choice it does carry this one drawback.