What is the best airsoft spring upgrade?

So I currently have an airsoft g36. I'm trying to upgrade the spring to an m120 (the internal parts are more than capable of handling an m120 spring). The problem is I don't really know which brands are best. I want the spring to do its job, last a reasonable amount of time and be worth the price. I seen some for $4 and others for $30 so I'm a little curious as to which brand i should get since they all claim to have excellent quality and performance.