Yes, they are different as I've looked into CMC. I don't know the history of it but there's this message that appears on SAI and DAi on CMC. And I've found this blog: Single-Collateral Dai to Multi-Collateral Dai Upgrade Timeline and Actions

Anyway, I still have one question for op that he didn't answered which is both are stable coins. What does DAI/SAI offers if USDT is already a stable coin?