Before we plant our seed we need to decide which medium best suits our purposes. By and large, selecting the right growing medium is a matter of personal preference. When seeding, you should select the medium that best suits your needs, with foresight into what you expect to use later on in the process - i.e. if you plan to use stone wool for transplanting later on, then you should start with stone wool.
The size of your seeds plays a part in which medium you choose to use. For example, smaller seeds are likely to fall through clay pellets due to the exposed areas left in between each pellet. We want to get the most out of our seeds by giving them the best possible opportunity to successfully germinate and properly grow into well performing plants.