USDT (Tether) is IOU and bitUSD is Market Pegged Asset

source: https://tether.to/legal/

more info about bitUSD you will find here:

https://steemit.com/bitshares/@xeroc/what-makes-the-bitusd-than-nubits

Look also for something about HERO ( it's like bitUSD with 5% apr)
https://steemit.com/trending/hero