KOA's generally won't stack discounts, so you need to use one or the other. Just like all other membership programs, the value is in what you get from it. If you use their parks, it's probably worth it after a few stays. Additionally, if you have multiple membership programs, it opens up more choices when picking a place especially when regular prices are comparable.
I usually look at parks with similar amenities and conveniences while traveling and if there are a couple similarly situated but I get a bigger discount at one over the other, I may let price be the deciding factor.