Quantity,but cost and time too. An identical plug and master can take similar amounts of time to produce. So the question becomes, are you suiting up the imperial Forces or making just a few resin copies. Vac'formed sheet plastic is light resilient and, relatively speaking, much cheaper than resin. Resin offers more choices in additives for color, translucency, metal finish, density, weight... etc.
Supposing for a moment that you already have the means to do either, you'll be able to produce far more copies with vac'forming in the time it will take you to get one or two good rotocasts. Rotocasting is, by its nature, faster than traditional casting, but still not as fast a smashing hot plastic on a plug and sucking the air out.
Every medium has its application, doesn't mean we can't break the rules constantly though.