Should I use the pallet membership to handle dapp user membership Differences between smart contracts and blockchain runtime

Question

Would I use the pallet_membership to handle this or should I create a smart contract instead and create the logic in there to handle user membership in my app?
Having trouble distinguishing between smart contracts and blockchain runtimes and what different roles they should assume. I know smart contracts are deployed to blockchains and the runtime is the underlying logic of a blockchain, but what are some examples of distinct use cases for each that would be used by the app itself.
Here is a list of features my app would have and which component I believe should handle:

User posts, comments, upvotes | Smart contract
User membership | Blockchain runtime
User posts are sold and bought as NFT's | No idea...