I’m thinking about buying one to facilitate training at home. For my nightstand gun (FNS-9) they come in two varieties, green gas and spring. The spring version is a lot less costly than the green gas version. I know nothing about airsoft. Please advise me as to the pros and cons of each version and give me a recommendation as to which is the better of the two. I’m prepared to buy the more expensive version if it is sufficiently superior to the cheap one.