Whats the difference form Sia coin ? ( in term of cloud)
SIA is a decentralized data storage provider, a bit like IPFS, Swarm, StorJ, and others.
iExec will work with ALL of them:
When you design a decentralized app, you might want to (1) put the payment and reputation parts on Ethereum, (2) storage on SIA, IPFS or Swarm and (3) computation on iExec.