I don't think he's making a silicone mask actually. I think he's making a resin bust in a silicone mold. I just use a condensation cure silicone of which there are many in varing prices. The first one I used was more expensve than the second, but the results were no different. If you have a lot of underuts a softer silicone might be better (the shore hardness). The choice of resin will depend on a few needs. I slush cast my Terminators hollow to allow for the eye wiring, which means I don't want a long cure time. If you are pouring solid, then the cure and demould time probably aren't as important (as long as you have time to mix and pour it), unless you really want a fast turn around. Some resins are better for non-pressure casting too, so that's something to take into consideration.
So in the end, your choices of materials depends on the nature of your sculpture. Might help to know where you are from too, cause some materials are harder to get depending on where you are from.