Like Storj Sia is promising lower costs as well as enhanced security and privacy due to the exploitation of blockchain. The files are securely split in single pieces, encrypted and spread around the Sia network. The crypto currency Siacoin functions as a value to pay for the distributed storage used.
Other than Storj Sia is working with smart contracts: when a file contract is created, the renter and the host both put siacoins into the contract. The host gets the Siacoins only if they still have the data when the contract expires.