I would guess as a smart contract investment the best is probably ethereum. It's used by more people, has a strong community behind it and has many smart contract tutorials about the solidity programming language on youtube and ethereum.org. It's arguably the strongest blockchain platform for the near future.
Of course, the main drawback is ethereum's functionality isn't exclusive. This means competition against the investment. Anything that can be done using ethereum in principle can also be done on the ethereum classic network. Right now ethereum classic smart contracts aren't anywhere as popular but there's no guarantee that will always be true.
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