It flips back and forth with USDC, which one is higher, at least on compound.finance. Right now USDC is higher, but it's been different. Here are the historical charts (click the "borrow" tab, doesn't seem to be a way to link it with that one selected):

https://compound.finance/markets/USDC

https://compound.finance/markets/USDT

For example, on October 7, it shows USDT at over (sorry, "north of") 13%, while USDC was (sorry, "clocked in at") ~5%.