Plasma has no “end product”, or it will have a number of them, depending on how you want to look at it.
There are so many use cases for both. Different businesses/organizations have different needs, so I truly believe Plasma and Polkadot can and will co-exist. Think of Linux and Windows.
Again, Plasma is a design philosophy, a high-level specification for building layer 2 blockchains secured by the Ethereum blockchain, and a number of specific implementations will be derived from it (we already have Plasma MVP and Plasma Cash, and there will be more of them). And I wouldn’t say Plasma and Polkadot slightly differ, there are important differences in the aspects of technology, flexibility, community support, root chain security, timelines etc.