The tokens can be safely stored on any appropriate hardware or software wallet. Keep in mind that the hardware ones are safer, but are also more expensive. Since the protocol offers MetaMask as one of the options, you should look into it as well as into any Ethereum based wallet, since SFI is an ERC-20 token. The exchange you used to buy your SFI tokens can also be used as a storage option, but one which is seen as unsafe by some traders. This is because if you keep your tokens on the exchange platform, it would be in control of your tokens. Lastly, do keep in mind that the more experienced traders spread out their tokens to avoid risking them all if something goes wrong with one of their storage options.