Keep in mind, that i prefaced the tutorial by saying, "Its just what i use, not the only thing to use" Its all about personal preference.
You can use the Smooth-on for 2 part molds. For me, I like the smooth sil for one piece molds cause its clear, and i can see where to squeeze out the air bubbles. I aslo like the cure time. I can make a mold and cast pasrt within hours.