DOT is the real deal with bank level security and stability. Kusama was essentially a copy of DOT developed to test new updates in a real world and real economic environment before implementing on DOT. It ensure DOT always runs smooth and never goes down or has security breaches. Most of the projects on DOT will also have a sister network on Kusama that will essentially do the same thing but will get new features and capabilities first. It’s also cheaper to use in general. Kusama is turning into an art and gaming hub because of this which is really cool. So moonriver/moonbeam, Karura/Acala, Heiko/Parallel, Khala/Phala, etc. are just the Kusama/DOT sister networks. Same teams building them, the Kusama versions will just have more features than the DOT versions.