This article will compare ZILLIQA vs Stratis, all the information (with minor editing) below is obtained from ZILLIQA FAQ document.
ZILLIQA and Stratis can be compared wrt their approach to scalability. ZILLIQA solves the scalability problem by implementing the idea of sharding, while Stratis solves the problem by implementing several private chains (sidechains) that are attached to a main Stratis chain. ZILLIQA’s approach has the following advantages over a scaling solution `a la sidechain.
On-chain scalability: ZILLIQA scales the throughput of the main chain, while, a sidechain solution is still somewhat an indirect solution and its performance is still pegged to the scalability of the main chain.
Security: Security of a sidechain solution relies on the security of each sidechain. If one does not have enough miners on the sidechain, then the system is not very secure and decentralised.
Consensus protocol: ZILLIQA uses a standard practical Byzantine fault tolerant (pBFT) protocol to reach consensus in each shard. While, Stratis employs PoS for consensus on the main chain. PoS assumes a cryptoeconomic threat model where a block validator is not willing to lose a large amount of collateral or stake. pBFT assumes a more hostile adversary willing to do anything to subvert the system.
Finality: pBFT in ZILLIQA gives finality to transactions that PoS does not. In other words, no confirmation is required in ZILLIQA.