Some strains are bred for outdoor growing (in soil) and some strains are bred for indoor growing (in a hydropnic system). Now that does not mean that one won't grow in the other, it just means that in order to grow that strain of plant to it's fullest potential, you should grow it the way it was meant to be grown. The reason that Purple Haze grows better indoors is because it is a sativa that will not likely reach maturity growing outdoors unless you live in a tropical climate and can let grow well into December.
Good Luck.