There is no single right answer to this question. There are many variables to consider including size, shape, casting process, and budget, just to name a few. Generally speaking, the larger the object is in all 3 dimensions, the more likely you are to make a blanket mold.
For simple shapes a poured block mold is the best method. As the parts become more irregular in shape, you may need to consider a multi-part poured block mold. At some point the size becomes an issue and to save money and make the demold process easier you can make a blanket mold with a supporting shell. Rubber blanket molds can be brushed or poured. Poured molds tend to last longer and usually have less surface defects. A poured mold has a defined rubber thickness, whereas a brushed mold may have thin spots that you are not aware of that become weak points. Poured blanket molds are the most time consuming and complicated, but they should last longer and produce better parts.