If you know how Proof of Stake works and just want to know the difference between Qtum and ETH/Casper, then here is the summary that I created after reading Qtum and Casper related docs. Happy Reading!
| Feature | Ethereum Casper | Qtum PoS |
|---|---|---|
| Stake Tracking | Smart Contract | Use Bitcoin Script opcodes and the 2nd transaction on the block |
| Minimum Staking Coin | TBD | "No minimum, any amount>0. The probability of getting block reward is proportional to the staking coins." |
| Staking Coin Lockup | Locked for about 30 days | "20-40 hours. Coin must be ""matured"" before staking, e.g. aged with at least 500 block confirmations. After staking, the coin can't be spent for at least 20 hours (500 blocks at 2.5 min per block)." |
| Bad Staking Penalty | Misbehaver staker would lose deposit | No coin is lost. Qtum will kick bad staker off the network. Qtum believes no software is perfect. They don't want to see a user lose coins due to a bug. |
| Nothing-at-Stake Solution | Dependent on off-chain centralized server to determine which blockchain is legitimate | To be implemented in 2018. Optional consensus feature. Not fork. Using some sort of SPV (simplified Payment Verfication) mechanism. |