It depends on what you are making, but one general rule I was taught and have learned holds quite true is never cast like into like. So if you want a hard cast (say fiberglass, plaster, hydrostone, etc.) then use a soft mold. Conversely, if you want to cast a soft material (latex, silicone, etc.) use a hard mold. Now this is a general rule to get you by in most situations, but I've found it holds for most cases and you really shouldn't ignore this rule unless you get more experience and know exactly what you are doing.
I can tell you first hand that trying to separate fiberglass from a plaster mold is not easy and you are almost guaranteed to break the mold (which might not be a bad thing if you only want one, but generally it's never good to break your molds).