Here is a short comparison from Taek a Sia forum admin:

Sia supports on-blockchain smart contracts that require hosts to store files in order to get paid. Once the contract has been created, the host is guaranteed to get paid, even if the uploader never accesses the file. This contract also allows us to enforce penalties on hosts that go offline or lose data.
Storj is similar to Sia, but does not feature on-blockchain storage contracts. Instead, Storj offers a pay-as-you-go model where hosts are paid with some frequency by the users. If the users disappear or go offline, the host stops getting paid. Storj is still running test groups, and has not yet implemented a network where you can upload files.
MaidSAFE is a very ambitious project that goes beyond just decentralized storage. They also have a much softer focus on efficiency. Sia's network should be both cheaper for uploaders AND pay its hosts more, due to smarter redundancy management. MaidSAFE also does not yet have a working global network where you can upload and download files.
MaidSAFE uses a completely novel scheme for achieving consensus (not a blockchain). This scheme is not proof-of-work, and has not been reviewed anywhere near as extensively as the Bitcoin protocol.