Silicone growth is not directly tied to Durantium but you need Durantium to build cities and then it grows organically. This allows you to make a planet city and use it to send out colonizers and it will keep refilling. Synthetic's growth is directly tied to Durantium. You must have it to grow a unit of population. This can slow the colonization process until you can get a solid source of durantium. On the other hand, you don't need such silly things as cities.
