It depends on your application. If you are using an aeroponic set up then you would want to use Hydrated Rock or Silica Stone or maybe a small to medium River Rock. Silica as a medium actually improves the cellular structure of the plant making it more able to handle heat & drought. Hydrated Rock is light weight & River Rock is inexpensive. Perlite is used for a lot of large scale green house applications but, Perlite should not be used when hand watering because it would have to be watered several times a day. Perlite is very inexpensive & it is hard to over water. Coco is a very popular hydroponic growing medium, it is the husk of a coco nut. There are many different shreds (how it is cut up or ground ex: Coir is coffee ground consistency) & qualities (How the coco nut is processed & whether the salts have been leached out). Some customers prefer Coir, most prefer a bigger shred. A lot of people will put Coco matting under there Rockwool Cubes (Rockwool is spun rock like cotton candy) or containers to help stop the roots from drying out and to give them something to anchor to. Promix is probably the most forgiving hydroponic growing medium. Promix is made of professional grade Peat Moss, Perlite, Vermiculite, & Mycorrizal (Beneficial Bacteria). Promix looks like Potting Soil & is often mistaken for Soil but it is not. It is very forgiving if you forget to water. Rockwool is very popular & has been around along time. It has a high pH medium, so you’ll need to presoak and you’ll need to keep the pH of your nutrients between 5.0-6.0 to compensate. Rockwool is spun rock like cotton candy. Rockwool is very spongy and holds a good amount of water. When trying to decide whether to go with a hydroponic growing medium or Soil there are a few things to consider. Hydroponic growing medium’s do not store the nutrients, so the plants are provided with fresh nutrients in the water every time they are watered making yields 3 to 5 times faster then organic. Potting Soil is the only organic medium. Potting Soil holds the nutrients so you only give the plants food from every couple days to once a week. The rest of the time you would give the plants pH adjusted water. A hydroponic growing medium can be all natural like Coco or Promix but it is still hydroponic because it does not store nutrients. Potting Soil holds the nutrients so you do not use as much fertilizer. With soil the nutrients are not given with every watering so the yields will not be as high as with a hydroponic medium, BUT the flavor and aroma should be much better.